11.x ABI compat isn't the headlining reason to retire, necessarily.
In the past this has been more like "Giant locked filesystem" or "Driver not migrated from old API" or simiar

int

This message needs to be more along the lines of "This hardware no longer relevant to FreeBSD" or "This wifi driver doesn't support FooBar crypto" or some other minimum bar.

11

This was copied from an in-tree one, but indeed "Giant locked filesystem" is probably a good canonical example

Here's some examples from the tree, as they'd appear in dmesg:

Deprecated code (to be removed in FreeBSD 13): pccard removed, an doesn't support modern crypto
Deprecated code (to be removed in FreeBSD 13): pccard removed, an doesn't support modern crypto
Deprecated code (to be removed in FreeBSD 13): amr(4) driver
Deprecated code (to be removed in FreeBSD 13): FreeBSD 11.x ABI compat
Deprecated code (to be removed in FreeBSD 14): FreeBSD 12.x ABI compat
Deprecated code (to be removed in FreeBSD 13): drm2 drivers
Deprecated code (to be removed in FreeBSD 14): esp(4) driver
Deprecated code (to be removed in FreeBSD 14): PC Card to be removed
Deprecated code (to be removed in FreeBSD 14): mly(4) removed
prefixed with the device name/number for gone_in_dev of course

IMO something short like "ISA drivers", "obsolete hardware" etc. is what we should suggest

sys/dev/mly/mly.c
339: gone_in_dev(dev, 13, "");

sys/dev/twa/tw_osl_freebsd.c
416: gone_in_dev(dev, 13, "twa(4) removed");

sys/dev/iir/iir_pci.c
339: gone_in_dev(dev, 13, "iir(4) removed");

I think this one probably should not change because it's literal console output.

Added Ns here