About 6 years ago I took ownership of a very complex subsystem at work in order to fix a bunch of issues. Things started working after I fixed them, so I moved on to other things. No one else stepped to maintain it, so work done on it was minimal since then. Bugs kept appearing (the rest of the system changes a lot), and I still get emails with bug reports and/or people trying to understand the code asking me for training or even worse: patch review requests. I'm not even in the same team anymore!
Here's a magical phrase "Sorry I really don't have time to help you with this". As much as it's an asshole move, the people asking for your help are being paid to work on it and you're not, so unless you're trying to garner brownie points for mentoring, you need to be clear about your boundaries.