Doing this backward compatible would probably require some cooperation from RIOT. I guess changing the XFA section name (e.g. from .roxfa.shell_commands_xfa.5 to .roxfa.shell_commands_v2_xfa.5) and providing both ptr and element would work, assuming that the linker would garbage collect the ptr in the newer version.

That seems to indeed just work

The garbage collection does not work here, so while this does allow adding shell commands in a way that works with both pre and post !20958 RIOT, it will not reduce the overhead.

I guess this is acceptable mid-term? We could drop the ptr for backward compatibility in the long run.

Dropping this on the long run sounds fine. This is shell commands, and a shell is not what you do when you're starved for every single pointer in RAM.