I will say that usually it is easy to move teams, but if the expertise of the team veers strongly from your background (web dev wants to work on C++) they might ask if you want to do 20% first to get a taste. Also can help convince a team to take a more junior hire, e.g.
I’m not sure if it’s ever been an explicit requirement and if you have strong performance reviews it’s usually pretty well-oiled.
Real 20% time though I don’t think I’ve ever encountered.
Maybe if you work on a team without a high-traffic product (internal tooling?).
YMMV, that’s all I’m saying. Don’t come in expecting it or you’ll be disappointed. And that’s resonated with other coworkers of mine on other teams, too.
Clearly it's not that easy.