hello 💜 i'm not really in the position to say what is 'allowed' and what is not, because i don't have authority over a single thing lol but i can give you my view based on personal experience.
in my opinion, when it comes to leaving comments on fics, it's best to stick to the basic rule of 'if you're not going to say something nice, don't say anything at all'.
if you read something and you don't like it, you can always exit out of the fic and read something else. leaving a negative comment isn't going to make the fic any different - it's just going to make someone feel bad about themselves and their skill.
writing fic is a passion project, and we do it because we want to share our writing with people who love the same media/characters as we do. i would encourage you to approach fic with the same mentality as if a friend gave you a gift. even if you didn't like the gift, you would never tell the friend that because it would probably hurt their feelings, right? the same applies here. you should absolutely use filters, read tags and summaries to give yourself the best reading experience possible and exit out of fics if you don't like them, but don't take away someone's pride in their own work, or make them sad.
i hope this helps (genuinely). when i first migrated from the wild world of ff.net i wasn't sure of the etiquette either (from comments left on my own fics - it was a lawless place 😂)
