This mod doesn't affect slabs, stairs and walls from the More Slabs Stairs & Walls mod, and I think it would be awesome if did, as this mod is just focused on adding new slabs/stairs/walls from already existing vanilla blocks.

If it isn't working on a mod, it's because they aren't using the vanilla classes. The only way to make it work would be to have a compatibility mod for it.

what's weird is that according to the source code they ARE using the base classes.

So, I've been having issues with Additional Placements not working with a lot of mods that it should be working with (i.e they all extend SlabBlock) seemly at random. I've been testing on Minecraft 1.21.1 - NeoForge, with Additional Placements 2.1.0 and 2.2.0.

I started by modifying com.firemerald.additionalplacements.Registration.tryApply() to show debug information about why certain blocks aren't being applied, and found that for some mods this method is simply never fired.

Tracing back further I found the real bug here: com.firemerald.additionalplacements.common.CommonModEventHandler.onBlockRegistry()

@SubscribeEvent
public static void onBlockRegistry(RegisterEvent event)
{
	if (event.getRegistry() == BuiltInRegistries.BLOCK)
	{
		List<Pair<ResourceLocation, Block>> created = new ArrayList<>();
		BuiltInRegistries.BLOCK.entrySet().forEach(entry -> {
			ResourceLocation name = entry.getKey().location();
			Block block = entry.getValue();
			Registration.tryApply(block, name, (id, obj) -> created.add(Pair.of(id, obj)));
		});
		created.forEach(pair -> Registry.register(BuiltInRegistries.BLOCK, pair.getLeft(), pair.getRight()));
		AdditionalPlacementsMod.dynamicRegistration = true;
	}
}

The issue here is your scanning the Block Registry, while Blocks are still being registered. So mods which were randomly registered before yours work, while those the register afterwords don't. That said, I'm not sure how to fix this, since you need to register you new blocks here, which need that information. Last time I was modding, back in 1.12.x, Forge had a postInit() method for this sort of thing, but I'm not seeing anything like that here.

I saw an old post for 1.19 about the registration order being alphabetical, by modid, so I tried changing you modid to zzz_aditionalplacements just to see what would happen...and works for some mods(at least my test mod sandrosextras), tho I'm not sure why, and I doubt that's right. supplementaries, and itsalive come after additionalplacements and they work either way, and new_slab_variants still doesn't work. All of these mods either use SlabBlock directly or extend it.

i use a mixin to dynamically add new ones as blocks are registered later on during the phase - see MixinMappedRegistry - so perhaps that's failing for some reason

Ok thanks, I'll look into this some more; still getting used to mixins. Still tho, the issue with these mods is that Registration.tryApply() is not being called...for some reason.

Ok, so yea your mixin is not applying in 1.21.x since MappedRegistry.registerMappings() no longer exists in 1.21. I believe it needs to be changed to MappedRegistry.register():

@Inject(method = "register(ILnet/minecraft/resources/ResourceKey;Ljava/lang/Object;Lnet/minecraft/core/RegistrationInfo;)Lnet/minecraft/core/Holder$Reference;", at = @At("RETURN"))
private void onRegisterMapping(int id, ResourceKey<?> key, Object value, RegistrationInfo regInfo, CallbackInfoReturnable<Holder.Reference<?>> ci) {
    ...
}

Tested working with Oh The Biomes We've Gone, and a sample mod I made for testing this issue. This should fix More Slabs Stairs & Walls on Fabric, tho I've only tested NeoForge. It also mostly works with New Slab Variants; although, some of their blocks now throw:

[13:26:31] [modloading-sync-worker/WARN] [Additional Placements/]: Generation type additionalplacements:slab cannot generate for new_slab_variants:oak_log_slab as it already contains the following properties that would be added: 
[13:26:31] [modloading-sync-worker/WARN] [Additional Placements/]: [axis]
[13:26:31] [modloading-sync-worker/WARN] [Additional Placements/]: Add it to the blacklist inside additionalplacements-startup.toml to stop this message from appearing in the future.

which is a separate and unrelated issue.

This should fix More Slabs Stairs & Walls on Fabric, tho I've only tested NeoForge.

Fabric shouldn't have this issue, I don't use a mixin for this there - they actually have a hook for this kind of thing already, net.fabricmc.fabric.api.event.registry.RegistryEntryAddedCallback.event(BuiltInRegistries.BLOCK)

[13:26:31] [modloading-sync-worker/WARN] [Additional Placements/]: Generation type additionalplacements:slab cannot generate for new_slab_variants:oak_log_slab as it already contains the following properties that would be added: 
[13:26:31] [modloading-sync-worker/WARN] [Additional Placements/]: [axis]
[13:26:31] [modloading-sync-worker/WARN] [Additional Placements/]: Add it to the blacklist inside additionalplacements-startup.toml to stop this message from appearing in the future.

god damnit, I'm gonna have to rename properties to avoid this one.

Fixed the mixin issue in:
1.21: b02c481
1.21.3: 9a8c9e3
1.21.4: 0a749aa
And renamed the blockstate properties to avoid the conflict in:
Forge 1.16.5: d4a7849
Forge 1.18.2: fd598a4
Forge 1.19.2: 2a90734
Forge 1.19.4: 3cfe35a
Forge 1.20.1: 7bb2e09
Forge 1.20.2: c8a1c26
Forge 1.20.4: 8d9a800
NeoForge 1.20.4: 05e1050
NeoForge 1.21: b02c481
NeoForge 1.21.3: 9a8c9e3
NeoForge 1.21.4: 0a749aa
Fabric 1.18.2: 2dad477
Fabric 1.19.2: c21a354
Fabric 1.20.1: c022593
Fabric 1.20.2: 847cdf3
Fabric 1.20.4: bb6d96c
Fabric 1.21: fb1feeb
Fabric 1.21.3: 7067acf
Fabric 1.21.4: effcb60