original submission:

> This promises to have been broken for a long, long time.

Since at least groff 1.19.2, I can confirm.  But it's such an edge feature I bet it never worked -- suggesting that one valid solution is to deprecate the register in groff.  (It's hard to argue a thing is critical to back compatibility if it hasn't worked in 30 years and no one has ever noticed.)

Input:

.\" groff calls the PostScript device's default font "TR'.
.\" DWB 3.3 troff called it simply "R", because it didn't group
.\" fonts into families.
.ie \n(.g .ds ft TR\"
.el       .ds ft R\"
.bd \*(ft 3
.tm by name: .b=\n(.b
.bd 1 3
.tm by position: .b=\n(.b

DWB 3.3 troff output:

$ DWBHOME=. ./bin/troff ./embolden.roff >/dev/null
by name: .b=3
by position: .b=3

groff 1.22.4 output:

$ groff EXPERIMENTS/embolden.roff
by name: .b=0
by position: .b=0

groff Git HEAD output:

$ ./build/test-groff -ww EXPERIMENTS/embolden.roff
troff:EXPERIMENTS/embolden.roff:6: warning: numeric expression missing
by name: .b=0
troff:EXPERIMENTS/embolden.roff:8: warning: numeric expression missing
by position: .b=0

(The -ww flag makes no difference in groff 1.22.4.  groff Git complains more.)

This promises to have been broken for a long, long time.
-verbatim-