the TLDR; many websites care including patreon, anyone that handles money including payment processors care
an example someone using patreon makes art of yuffie getting DP'd by cloud and barret, someone posts on twitter, it gains traction people complain its underage porn. patreon will remove it cos they dont want their daddy moneybags to cancel their ability to make money.
The "law" about depiction of underage persons or whatever it specifically is kinda needs to be solidified to either only be about real people or include fictional characters cos my understanding is at the moment it doesnt, so it just blanket covers everything and everybody plays it safe cos they don't want to get sued or lose money. Japan is seemingly one of the few places that "doesn't care" in a sense its something to do with them not wanting to encroach on the "freedom of artists" i could be wrong since i only saw this mentioned randomly in a post once about why they have so much underage stuff on the other hand they still censor their porn for the last 80 yrs and that doesnt look like to change anytime soon either
If I'm wrong about any of this feel free to correct me cos i obviously i don't know shit legally speaking.