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If the mod is open source, then it is completely fine, and likely one of the reasons the mod author made their mod open source to begin with. If you release a mod, perhaps include the authors of open source mods that have given you help/inspiration in your credits/thanks/readme.

The method you are using returns an unmodifiable list in its default implementation so you can't clear it. If you want to modify the actual data, you will need to use an access transformer to make the Item.category field public https://forge.gemwire.uk/wiki/Access_Transformers But there is no guarantee the Item actually uses that field. An item can override getCreativeTags() to do something else (vanilla items don't do this). You should probably also be aware that Mojang are changing how this works in 1.19.3 https://www.minecraft.net/en-us/article/minecraft-snapshot-22w42a