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Feature requests happen in issues too, and this the source code for a book, so an example seems appropriate

It definitely depends on the preferences of the project but GutHub issues for feature requests is fairly common. For a long time, GH didn't have a better place for them.

These days the "discussions" feature can be enabled and some projects prefer feature requests be added there. This repo has it turned on, but idk what their preference is for requests:

https://github.com/curl/everything-curl/discussions

Those are the type of issues they track: https://github.com/curl/everything-curl/labels