When I search for how I can keep a material when removing an object, I only get answers to the opposite question: How to remove the material. I want to remove the object but want to keep the material. Is that possible?
The material isn't deleted with the object; the former will only have 0 users and hence will be deleted on reopening the file. To avoid that use Fake User option:
Gives the material a ‘fake user’, to keep the material data-block saved in the blend-file, even if it has no real users.
More info on datablocks and fake user - What does "users" means in term of datablocks ?