honestly, the term 'clean' is where things get tricky right from the start. Different readers have wildly different definitions. Some folks just want zero smut, others want no major character death, and some are looking for absolutely no angst or conflict at all. That makes platform recommendations a bit of a minefield.
For my money, Archive of Our Own remains the gold standard for finding what you want, precisely because of its tag system. You can exclude anything you don't like. Filter out 'Explicit', 'Graphic Depictions Of Violence', and even specific relationship dynamics. You have to do a bit of tagging work yourself, but once you set up a search, you can find tons of gen fics, fluff, and post-pacifist route stories that are wonderfully soft.
That said, the quality of 'clean' content really depends on the writer community at any given time. I've noticed that on Tumblr, a lot of artists and writers create super sweet, comic-style drabbles and headcanons that are inherently lighthearted. You just have to follow the right blogs, and the content tends to be very wholesome by default.
Wattpad can be a gamble. The algorithm pushes certain tropes, and a lot of Sans x Frisk there leans into the more dramatic, soulmate-y stuff that isn't 'explicit' but can get pretty intense emotionally. I'd say AO3 with careful filtering is your most reliable bet, followed by curated Tumblr blogs if you're okay with shorter-form content.