That would be really nice to be able to toggle certain changes.
For example UI elements. I have a mod in my pack that changes effects HUD, but it gets overwritten by your mod. Also some of the menus feel just more comfortable in their Java state, so it would be really convenient to choose which elements of the game your mod modifies.
Last but not least, mods that add their own menus and HUDs are not affected by your mod. It makes them look unnatural. My previous idea could solve it.

Well, there are some other things that cannot be toggled off in Legacy4J, including, but not limited to:

• Removing the mod's GUI scale override so that it matches the scale in Minecraft's Video Settings.
• Disabling legacy creative flight
• The mod could have a client-side "fair-play version" that only changes the GUI and does not have any server-side features or registry additions. This version also would remove any features that could get you banned from servers.
• The mod adds three new items (legacy:water, legacy:lava, legacy:shrub) to the registry. This can cause issues when Legacy4J is removed due to errors like "Forge Mod Loader has detected missing registry entries". Thes items can be split into a separate mod to resolve the issue. By the way, these are items that were formerly present in Java Edition before eventually being removed.