I have to disagree on this take. Almost all code dumping is useful since you can always learn something from it (either a new technique or stuff TO NOT DO...). What is important is to state the status of the project. If someone is still using the project and having difficulties with it even if you added a disclaimer, then it is their problem, and they should not expect any support (unless your license requires so)...
> But if you get a pull request, then someone else has spent time on your project, and just throwing that away is not good.
Sure, but once again communication is key here. I absolutely understands maintainers that do not have a look at merge requests but let them open. It allows anyone with the same problem to easily search in opened issues/merge requests and find the patch, which may help them (even if it is not in the "main" remote).
> I ended up completely dropping out of that project as a way of preserving my mental health. In fact, I dropped out of all my (F)OSS projects, and I even deleted some of my “code dump” projects on GitHub.
Hope you feel better now.