"or similar" - I mean an open-source free license to use, that is not encumbered by patents. GPL was the first license name to pop into my old-man brain.

It could still serve as a reference implementation.

Not only a GPL reference implementation, GPLv3 would preclude patents on any improvements.

I don’t see how that follows?

GPLv3 has a clause that says if you release anything under the license then you have to provide a cost free license to anything that is patented, if you aren't able to provide a license then you aren't allowed to release it under GPLv3. Any modifications to a GPLv3 licensed product has to be released under GPLv3.

So yes you are correct, but in effect it might as well be patent free as far as a 3rd party end user is concerned as they have been provided what is in effect a safe harbour.

It could still be useful if the patents were part of a defence patent portfolio. (Ideally I'd abolish software patents altogether, or set a hard 10 year limit on them together with some kind of mechanism to curb the impact of add-on and submarine patents.)

Yes, it can be used defensively. If you breach the GPLv3 then in effect you also lose your patent license. The GPLv3 also forbids you making an opposing patent claim, so in effect it is defensive.