User blog comment:MisterWoodhouse/The first Unified Community Platform release is now live for NEW wikis/@comment-36153576-20200319162801/@comment-37336800-20200319163909

As far as I can tell there is no toggle at this point (iirc I saw a Fandom staff member say it's a known issue), but you can just go to  on a UCP wiki and you'll get a talk page instead of being redirected to the message wall as is the case on non-UCP wikis with message walls.