c/feedback is the official spot. For more urgent matters, tags/reports/DMs to admins can also work.

Feature requests can be tough because we (currently at least, open to volunteers) have very limited development resources, but feel free to ask, there's always a chance the feature is something that can be done with a config change or a bot or a userscript, or that another user will come along and go "I reckon I could implement that."