Deprecate .Tn and .Ux, and make it clearer that .Bt and .Ud are deprecated.
Do not use these macros in new documents, they provide no value.
Instead, usually no macro and no markup is needed at all.
Of course, they remain supported for compatibility with existing manuals.

Jason McIntyre (OpenBSD), Thomas Klausner (NetBSD) and
Franco Fichtner (DragonFly) are OK with this documentation change.