sshd_config.5 line 1624 mentions "%%, %f, %h, %t, and %u" as accepted tokens, but does not mention "%k", which is also supported (line 1020 of auth2-pubkey.c). It seems this was lost during the reorganization in v1.235 of sshd_config.5.
Fixed, thanks.
Close all resolved bugs after release of OpenSSH 7.7.