While working on Item Descriptions, I've found that spawn eggs, unlike similar groups of items like normal eggs, do not have their own organizational tag. This means in order to detect them, I need to use instanceof, which is a functional enough workaround, but is less than ideal. Other applications of this tag could be for finding them by searching via tag like in the Creative menu's search or through EMI.

I think spawn eggs is the one kind of item that you really should use instanceof to detect. Anything relating to spawn eggs really is about the specific functioning of spawn eggs item class itself. For example, someone adding minecraft:egg to the spawn egg tag could mess up mods reading the tag in unexpected ways if they were expecting spawn egg behaviors

One more thing to consider. Unlike other tags, this one would require every single mob mod to opt in and define every single one of their entities into the tag to make it useful. If even one large mob mod doesn’t use the tag, the tag is incomplete and not very useful for your mod. But if you use the instanceof check, you are guaranteed to always work with all mob mods because spawn eggs is a standardized class everyone uses.

In this case, it’s best to stick with the most reliable option you got available tbh. But that’s my thought

It's one of the two hardcoded checks in the entire mod, honestly wanted to check if it'd benefit anyone else. I'm fine without it.