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Reformatting a Btrfs subvolume is a somewhat strange concept. They aren’t formatted to begin with.
I don’t think removing the subvolume and recreating it is a workaround. I think that is the correct and logical action to take.
I think creating a way to empty a subvolume in the installer may actually create more confusion.
Yeah, I agree. The problem is that the installer itself uses this terminology even for subvolumes. So if you want to mount a subvolume as /, it should probably check whether it’s empty and refuse when it’s not. But instead it says “it needs to be formatted”. What it could do is rename the “reformat” checkbox to “recreate” for btrfs, and automatically remove and recreate the subvolume (with the same name) before installation. Then it doesn’t need to check for emptiness and users don’t need to manually delete and create a new one. But currently it just uses a confusing messaging.
I wondered whether to explain all of this in the description and probably make it even longer and harder to read, but I decided to just go with the “reformat” word for everything. If people think it’s too confusing, I can try to rewrite it.