I think it would be valuable to display this at the top of the README, in the contributors guide for the docs, and also in the issue templates.

Consider what the maintainers ofrequests have in their docs here:

https://requests.readthedocs.io/en/latest/dev/contributing/?highlight=feature%20complete#feature-requests

Requests is in a perpetual feature freeze, only the BDFL can add or approve of new features. The maintainers believe that Requests is a feature-complete piece of software at this time.

One of the most important skills to have while maintaining a largely-used open source project is learning the ability to say “no” to suggested changes, while keeping an open ear and mind.

If you believe there is a feature missing, feel free to raise a feature request, but please do be aware that the overwhelming likelihood is that your feature request will not be accepted

I want to add few points explicitly with what Tom mentioned:

1. Bug fixes
2. Long standing design limitations / implementation detail improvement for better dev exp without breaking much
3. Not exposing any new API surface unless needed for any existing features support: explicitly decided only by Old and new maintainers. But community inputs are welcome.
4. Improving test and CI infra.