I tend to do something similar, but I always at least open an issue/PR (depending on the amount of work I did on the fork) and just list the things I did, why I did them, and some useful docs/links that can help someone pick up where I left off.

on multiple occasions I've gotten emails about PRs I made and self-closed with someone thanking me for the start of the general solution, and that finished it up and made it into a full PR.