Maybe this would be more useful as a warning message:

"Modifier type cannot be added to object '%s'" -> "Object '%s' does not support %s modifiers", ob->id.name + 2, BKE_modifier_get_info(md->type)->name

Maybe that suggestion is a little too conversational, but something like that would be nice.

Nitpicky: I think "Copy the modifier from the active object to all selected objects" reads a bit better.

Besides LISTBASE_FOREACH I would suggest reorganizing this code a bit, mostly to make the extra checks for these modifier types more separate.

It's a bit easier to just put a diff here if you don't mind:

diff --git a/source/blender/editors/object/object_modifier.c b/source/blender/editors/object/object_modifier.c
index 9cab829b9bc..90ca3828ae0 100644
--- a/source/blender/editors/object/object_modifier.c
+++ b/source/blender/editors/object/object_modifier.c
@@ -1681,27 +1681,24 @@ static int modifier_copy_to_selected_exec(bContext *C, wmOperator *op)
       ParticleSystemModifierData *psmd = (ParticleSystemModifierData *)md;
       object_copy_particle_system(bmain, scene, ob, psmd->psys);
     }
-    else if (ELEM(md->type, eModifierType_DynamicPaint, eModifierType_Fluid)) {
-      DynamicPaintModifierData *pmd = (DynamicPaintModifierData *)md;
-      FluidModifierData *fmd = (FluidModifierData *)md;
-
+    else {
       BKE_object_copy_modifier(ob, obact, md);
+    }
 
+    if (ELEM(md->type, eModifierType_DynamicPaint, eModifierType_Fluid)) {
       ParticleSystem *psys_on_modifier = NULL;
 
       if (md->type == eModifierType_DynamicPaint) {
-        if (pmd->brush) {
-          if (pmd->brush->psys) {
-            psys_on_modifier = pmd->brush->psys;
-          }
+        DynamicPaintModifierData *pmd = (DynamicPaintModifierData *)md;
+        if (pmd->brush && pmd->brush->psys) {
+          psys_on_modifier = pmd->brush->psys;
         }
       }
       else if (md->type == eModifierType_Fluid) {
+        FluidModifierData *fmd = (FluidModifierData *)md;
         if (fmd->type == MOD_FLUID_TYPE_FLOW) {
-          if (fmd->flow) {
-            if (fmd->flow->psys) {
-              psys_on_modifier = fmd->flow->psys;
-            }
+          if (fmd->flow && fmd->flow->psys) {
+            psys_on_modifier = fmd->flow->psys;
           }
         }
       }
@@ -1749,9 +1746,6 @@ static int modifier_copy_to_selected_exec(bContext *C, wmOperator *op)
         }
       }
     }
-    else {
-      BKE_object_copy_modifier(ob, obact, md);
-    }
 
     WM_event_add_notifier(C, NC_OBJECT | ND_MODIFIER, ob);
     DEG_id_tag_update(&ob->id, ID_RECALC_TRANSFORM | ID_RECALC_GEOMETRY | ID_RECALC_ANIMATION);

I actually think this whole section of moving the particle systems should probably be split into a separate function.
And adding a comment to that function about the big picture of why this special case is necessary would be even better

I'd like to avoid using needlessly terse names like npsys, just expand it something like psys_new

Use LISTBASE_FOREACH to iterate over ListBases, it's much more readable and enforces smaller scope for variables

This should be title case (Copy to Selected)

I know the existing menu already does this, but I think the double icon here is a bit ugly. Personally I would skip the icon for this operator.

/home/hans/Documents/Blender-Git/blender/source/blender/modifiers/intern/MOD_ui_common.c:266:32: warning: declaration of ‘ob’ shadows a previous local [-Wshadow]
  266 |   CTX_DATA_BEGIN (C, Object *, ob, selected_objects) {
      |

I suggest using ob_iter for this name, same for gpencil modifiers.

Since this is the third time I'm seeing this check in this patch and it seems like a reasonable check to have in BKE, I would add this as BKE_object_supports_modifiers() to object.c

Instead of counting the entire total just to check if it's greater than one, just use a bool and break if one of the selected objects doesn't equal the active object.

Just because it's not super clear from the name, I would suggest adding a comment:

/**
 * \return True if the object's type supports regular modifiers (not grease pencil modifiers).
 */

This should be RPT_ERROR, because the operator doesn't work in this case.

If the modifier uses particles

Okay, I did some testing with the Fluid modifier that left me a bit confused about the goal of this function. I copied a liquid fluid modifier with particle settings to another object. The modifier was copied, but the fluid didn't move when I started playback, and the particle system wasn't copied to the new object. Either way, a description of the situations this is supposed to handle and some thorough testing would probably be best.

This is checked by the caller with

if (ELEM(md->type, eModifierType_DynamicPaint, eModifierType_Fluid)) {
  copy_or_reuse_particle_system(C, ob, md);
}

So instead of the previous else if I would recommend using if and BLI_assert(md->type == eModifierType_Fluid

Check psys_on_modifier == NULL here and return early, that way the rest of the function loses one level of indentation.

RPT_ERROR here too.

See the note in MOD_ui_common.c about the poll function, which I don' think makes sense in this case, since the modifier is not edited.

This should become the poll function for this operator (same for the grease pencil modifiers). The poll currently checks if the modifier itself is editable, but the operator doesn't actually edit the modifier, it copies it.

Using BKE_object_support_modifier_type_check would be better, that way if you're in the menu of a bevel modifier and have 4 other volume objects selected, it will be disabled.