I think this is kinda the best way. If you're a manger who used to code, do the sort of tedious tech debt stuff for your team. Update dependencies. Build small tooling improvements. Do the sort of stuff your devs probably want to do but have higher priority work that will get in the way. That's likely work that doesn't require you to have deep knowledge of how everything works, but still provides value.

If your project is complex enough that's not an option, then write onboarding docs and other technical stuff. IMO, the manager shouldn't be writing code much, but they should always keep a running version of the project. They should be able to run tests, confirm that PRs function locally, just keep a basic attachment to things.