My team when I first joined had something like this; we called it a 'spike' day. Basically a full day of time where we could learn about something new, maybe hack on a non-necessary feature or even personal project for a few hours a week.

But sadly over the past few years we've kind of just done away with it. We don't allot time for it specifically anymore. We think about it from time to time and wish we still had the bandwidth for it.

But it just doesn't happen anymore.

It's a tough thing to build into a team's structure, I think, and in such a way that it's sustainable.