what I want to do is the exact opposite of what people normally do.
I want to create GitHub issues that apply only to a particular branch I'm currently working on. Ideally, they would not be visible when browsing other branches of the repo. Even more ideally, they would only be visible to the repo owner (me).
To give a little more context, I'm working on the next version of a rather large macOS app, and the new issues I want to create would only be relevant to this new version, not to the master branch (old app version). That is why I want these issues to only be associated with a specific branch, not with the repo as a whole.
I know that I can associate an issue with a "project" or "milestone" but I'm looking for something different.
Any ideas ? Thanks.
This is not possible. Issues are repository wide. You can add in the title or body of the issue that it is for a specific branch. Or you can use labels saying what branch it is for. You can also make a branch for the issue on the repo or a fork of it for even more of a difference. And for when the PR is made you can link it.