I just recently did an interview with Gitlab and they do this. They have a codebase that is obviously not something they monetize or use themselves for anything other than interview purposes (for the ones worried about stolen labor). They make a PR for the codebase to add a new small feature, and assign you as a reviewer.

I quite liked it, though I wasn't a fan of how bad the PR is itself, since I ended up having way too many comments, some of them being massive "wtf is this and why would you ever do it this way?" Types of things that IRL I would've refused to review without a proper rewrite.