Address review:

- more docstring
- modified doco
- deprecate old boundary parameters

framework/doc/content/source/meshmodifiers/ElementSubdomainModifier.md

@@ -2,7 +2,7 @@
 
 ## Overview
 
-The `ElementSubdomainModifier` modifies an element subdomain ID. This is inherited by other mesh modifiers, such as [CoupledVarThresholdElementSubdomainModifier.md] and [TimedSubdomainModifier.md], which implement different criteria for a subdomain ID to be modified.
+The `ElementSubdomainModifier` modifies an element subdomain ID. This class is inherited by other mesh modifiers, such as [CoupledVarThresholdElementSubdomainModifier.md] and [TimedSubdomainModifier.md], which implement different criteria for a subdomain ID to be modified.
 
 The `ElementSubdomainModifier` can model
 
@@ -25,7 +25,7 @@ Consider a unit square domain decomposed by a vertical line $x=0.25$. The elemen
 !alert note
 If the moving boundary is defined completely around a subdomain, or between subdomains, then [SidesetAroundSubdomainUpdater.md] may be more useful to use in conjunction with the `ElementSubdomainModifier`, rather than using the `moving_boundaries` and `moving_boundary_subdomain_pairs` parameters in `ElementSubdomainModifier`.
 
-The change of element subdomains will alter the definitions of certain sidesets and nodesets. The parameters `moving_boundaries` and `moving_boundary_subdomain_pairs` can optionally be used to modify the corresponding sidesets/nodesets over the elements that change subdomain. The names of the boundaries are specified in `moving_boundaries`, and the pair of subdomains that each boundary in `moving_boundaries` lies between must be specified in the corresponding `moving_boundary_subdomain_pairs`. The element side from the first subdomain of the pair is added to the boundary.
+The change of element subdomains will alter the definitions of certain sidesets and nodesets. The parameters [!param](/MeshModifiers/ElementSubdomainModifier/moving_boundaries) and [!param](/MeshModifiers/ElementSubdomainModifier/moving_boundary_subdomain_pairs) can optionally be used to modify the corresponding sidesets/nodesets over the elements that change subdomain. The names of the boundaries are specified in `moving_boundaries`, and the pair of subdomains that each boundary in `moving_boundaries` lies between must be specified in the corresponding `moving_boundary_subdomain_pairs`. The element side from the first subdomain of the pair is added to the boundary.
 
 If the boundaries provided through `moving_boundaries` already exist, the modifier will attempt to modify the provided sidesets/nodesets whenever an element changes subdomain ID. If the boundaries do not exist, the modifier will create sidesets and nodesets with the provided names.
 
@@ -80,7 +80,7 @@ An auxiliary variable $u$ is defined over the domain, with initial values of 1,
 
 !media large_media/mesh_modifiers/element_subdomain_modifier/orig.png style=float:center;width:100%; caption=The default behaviour of the modifier is to reinitializate all elements that change subdomain ID.
 
-However, if a list of subdomains (IDs or names) is provided through the parameter `reinitialize_subdomains`, the reinitialization only occurs if the element's new subdomain ID is in the provided list:
+However, if a list of subdomains (IDs or names) is provided through the parameter [!param](/MeshModifiers/ElementSubdomainModifier/reinitialize_subdomains), the reinitialization only occurs if the element's new subdomain ID is in the provided list:
 
 !listing test/tests/meshmodifiers/element_subdomain_modifier/reinitialization_into.i start=[MeshModifiers] end=[AuxVariables]
 
@@ -92,7 +92,7 @@ If an empty list is given in `reinitialize_subdomains`, then there is no reiniti
 
 !media large_media/mesh_modifiers/element_subdomain_modifier/none.png style=float:center;width:100%; caption=No reinitialization of any elements that change subdomain ID.
 
-Reinitialization can be further restricted by setting the parameter `old_subdomain_reinitialized` to `false`. The modifier will then additionally check the element's old subdomain ID. Reinitialization then only occurs if the old subdomain ID was not in the list provided in the parameter `reinitialize_subdomains`.
+Reinitialization can be further restricted by setting the parameter [!param](/MeshModifiers/ElementSubdomainModifier/old_subdomain_reinitialized) to `false`. The modifier will then additionally check the element's old subdomain ID. Reinitialization then only occurs if the old subdomain ID was not in the list provided in the parameter `reinitialize_subdomains`.
 
 !listing test/tests/meshmodifiers/element_subdomain_modifier/reinitialization_from_into.i start=[MeshModifiers] end=[AuxVariables]
 

framework/include/meshmodifiers/ElementSubdomainModifierBase.h

@@ -13,6 +13,9 @@
 #include "NonlinearSystemBase.h"
 #include "AuxiliarySystem.h"
 
+/**
+ * Base class for mesh modifiers modifying element subdomains
+ */
 class ElementSubdomainModifierBase : public ElementUserObject
 {
 public:
@@ -88,11 +91,13 @@ class ElementSubdomainModifierBase : public ElementUserObject
                                ConstElemRange & elem_range,
                                ConstBndNodeRange & bnd_node_range);
 
-  /// Determine if a node is newly activated
+  /// Determine if a node is newly reinitialized
   bool nodeIsNewlyReinitialized(dof_id_type node_id) const;
 
+  /// Reinitialize variables on range of elements and nodes to be reinitialized
   void applyIC(bool displaced);
 
+  /// Reinitialize stateful material properties on range of elements and nodes to be reinitialized
   void initElementStatefulProps(bool displaced);
 
   /// Range of reinitialized elements

framework/src/meshmodifiers/ElementSubdomainModifierBase.C

@@ -29,10 +29,10 @@ ElementSubdomainModifierBase::validParams()
   params.set<bool>("use_displaced_mesh") = false;
   params.suppressParameter<bool>("use_displaced_mesh");
 
-  params.addParam<BoundaryName>(
+  params.addDeprecatedParam<BoundaryName>(
       "moving_boundary_name",
       "This has been replaced by 'moving_boundaries' and 'moving_boundary_subdomain_pairs'.");
-  params.addParam<BoundaryName>(
+  params.addDeprecatedParam<BoundaryName>(
       "complement_moving_boundary_name",
       "This has been replaced by 'moving_boundaries' and 'moving_boundary_subdomain_pairs'.");
   params.addDeprecatedParam<bool>(
@@ -177,6 +177,14 @@ void
 ElementSubdomainModifierBase::modify(
     const std::unordered_map<dof_id_type, std::pair<SubdomainID, SubdomainID>> & moved_elems)
 {
+
+  // If nothing need to change, just return.
+  // This will skip all mesh changes, and so no adaptivity mesh files will be written.
+  auto n_moved_elem = moved_elems.size();
+  gatherSum(n_moved_elem);
+  if (n_moved_elem == 0)
+    return;
+
   // Create moving boundaries on the undisplaced and displaced meshes
   //
   // Note: We do this _everytime_ because previous execution might have removed the sidesets and
@@ -186,13 +194,6 @@ ElementSubdomainModifierBase::modify(
   if (_displaced_mesh)
     createMovingBoundaries(*_displaced_mesh);
 
-  // If nothing need to change, just return.
-  // This will skip all mesh changes, and so no adaptivity mesh files will be written.
-  auto n_moved_elem = moved_elems.size();
-  gatherSum(n_moved_elem);
-  if (n_moved_elem == 0)
-    return;
-
   // This has to be done _before_ subdomain changes are applied
   findReinitializedElemsAndNodes(moved_elems);
 
@@ -531,6 +532,7 @@ ElementSubdomainModifierBase::applyIC(bool displaced)
   _fe_problem.projectInitialConditionOnCustomRange(reinitializedElemRange(displaced),
                                                    reinitializedBndNodeRange(displaced));
 
+  mooseAssert(_fe_problem.numSolverSystems() < 2, "This code was written for a single nonlinear system");
   // Set old and older solutions on the reinitialized dofs to the reinitialized values
   setOldAndOlderSolutions(_fe_problem.getNonlinearSystemBase(_sys.number()),
                           reinitializedElemRange(displaced),