Yeah I recall asking for something akin to this ages ago. For me it was more about being able to take clean screenshots. That’s something the game engine already does automatically, so I’m sure it can’t be a huge task to optionally hide the UI.
Here’s my related post from 2015:
https://gozwift.zendesk.com/hc/en-us/community/posts/203595113-Clean-screenshot-no-UI-