That was a feature added later, mostly to address instance operators' fear for police intervention for enabling access to Japanese content(they say loli but even Japanese in 30s to 40s are legit mistaken even in court as underage, so not much to do with age, IMO more of specific makeup style or lack thereof)
There's also pawoo.net, which was operated by Pixiv (switched hands).
Pawoo is the go-to instance for (lolicon) artists banned from Twitter, and despite being the largest Mastodon instance is blocked by almost all instances.
The fear came from some European laws forbidding under-aged illustrations (typing this makes me die inside), so instances serving pictures from Pawoo may get into trouble.
I kind of don't like how "lolicon" is being used -- because pedophilistic orientation don't seem to correlate all that well to how likely people is banned from Twitter. There's a real pedo guy on it, who I've seen public about how arts don't substitute stuffs, who posts his... arts a bit jagged, and doing it for years, then on the other hands there are artists getting banned nth times this year posting an abstracted person with clean and continuous lines.
My theory is there's a set of curve parameters that evaluate to "legality" in a sigmoid response that has less to do with ages or even if it's depicting a human or an animal or not, like a picture of desert hills looks pornographic sometimes or how sumo wrestlers charging evoke no sexual emotion. That human curve scoring yet to be discovered is, to me, looks like how legality is determined worldwide on and off internet, so calling those drawings as loli or people doing those curves as lolicon is inaccurate in my opinion.
