From what I could see on his Pixiv the last 3 illustration he made looks very sus and AI generated, before he always managed to make good looking eyes who are similar, but on the last 3 illustration they just look weird and not proportional, also he completely change the lighting he used and its way too similar to those used in AI-generated pictures. For me there was a use of AI but maybe real artist here could help us to see if there was indeed AI use in that illustration
Edit : its not, I just checked is Twitter and he made step by step of that illustration.
Would it be productive for someone to make a “suspected AI” tag or would that get too out of hand with how subjective that would become?
The tag would only be short-lived, like tagme since it's just placeholder before someone found out if the art was 'drawn' using AI fully or assisted by AI. It's quicker and more efficient to ponder the image and search the source for traces of AI usage than creating a tag just for it to be replaced later.