Currently rear contains many files
with explicit GPL-2.0+ or GPL-2.1+ licensing notes
of the form

either version 2 of the License, or (at your option) any later version

or

either version 2.1 of the License, or (at your option) any later version

so that some (older) rear files are still "GPL-2.0+"
but some (newer) rear files are "GPL-3.0"
according to the current COPYING file in rear
that was changed from GPL version 2 (up to rear 1.17.2)
to GPL version 3 (since rear 1.18)
see the pull request #739