I'm not sure why your T would want you to cut online communication. Maybe if it's problematic for you? Or something you want/need to quit already? Or if it's a support forum--does she have a problem with the kind of info or contact you're getting there, does she think it interferes with therapy in some way?

Seems like something you should get her to clarify, for sure. Would I trust her, maybe, that would depend on a whole host of other factors, like what her reasoning is and whether she's proven trustworthy in the past.