The Unearthed Arcana content published by Wizards of the Coast generally comes with a survey a few weeks after that allows feedback on the respective article. However, for most of the current UA content, this survey is already over (e.g. the Mystic class), while the features haven't been published yet in an official rulebook such as Xanathar's Guide to Everything.
Is there an official place to provide further feedback?
The only possibilities I could think of are tweeting to the designers, or reviewing on DM's Guild - neither of which feel very promising.