I have no idea, though, why shortmess is no longer documented in 5.10.x (in 5.8.8 it was), despite the fact that it's still there and works... P.S.: Does anyone know if it has been deprecated (nothing related is mentioned in either the 5.10 or the 5.8 docs), or what else might be wrong with using it?

I do not know of anything wrong with shortmess. A quick glance at delta docs does not reveal anything. I recommend you submit a patch to add this back into the POD using perlbug. Either it will be fixed, or you should get a response explaining why it will not be fixed.

I have been planning to submit a patch for a minor POD bug in Carp, but I have been too Lazy.