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According to rms, SPDX has agreed to change their license identifiers to clarify GPLvN only vs. GPLvN+:

That's great news!
Björn Schießle @bjoern

@mikegerwitz I'm surprised, according to the spdx documentation they already support the '+' operator to identify 'or any later version' licenses.

@bjoern The issue was that using something like "GPL-3.0" isn't clear to the user---especially users inexperienced with licensing---that it won't be compatible with future versions of the GPL. When a user chooses a license for her project, there's nothing suspicious about "GPL-3.0".

If it's "GPL-3.0-only", though", users will wonder what the alternative to "only" might be.

@mikegerwitz Thanks for explaining. I see the point. Although I never understood the SPDX identifier as a list which helps you to chose a license for your project but rather as a machine readable identifier you add after you have chosen a license. A list which help you to chose the license should be much more descriptive than just a name, also gplv3-only would be to short in my opinion. Therefore I'm not sure if this change solves a real problem. But anyway, I got the motivation for the change.