[adtac]

Although bear in mind that open sourcing it might require you to spend some time on the project answering to bug reports and stuff. You might not even intend to add any new features, but if someone else comes in with a new PR, you'll probably (atleast I will) be inclined to review it out of courtesy. Do this for enough projects and you'll be spending a considerable chunk of your time on the weekends on this.

However you might, of course, choose to ignore everything: bug reports, new PRs, users asking for help, everything.

[mikekchar]

I wouldn't worry about it. I have tons of free software projects that nobody has ever asked about :-) My one project that had a significant number of users (a Japanese language drill program) I recommended that people move to Anki. I occasionally get people asking about various things, but it is never more than I can handle in an adhoc way. I suspect most other people will be the same. Wildly successful side projects are the exception, not the norm.

[toyg]

Just mention in README that there will be no further updates, and "please fork it yourself".

[Hedja]

You can disable issue trackers and PRs and let them know in the README.