imap_admins, lmtp_admins, [service]_admins?

[ellie timoney]

Bits of our documentation refer to these various service-specific admins
settings, and googling a bit I see a bunch of people using them in the
imapd.confs they post when asking for help with stuff.

But I went looking for where/how they're implemented (to see what was
needed to support the same sort of thing for backups) and-- this
functionality doesn't seem to exist anywhere in our source?  I was
expecting to find something building a config key from a service name
plus "admins" and then using config_getoverflowstring to check for it
(like how the channel-specific config keys for sync_client work) but...
nothing.

Is this a thing that used to exist, and we've removed it at some point
for some reason?  Is it a thing that was planned and documented, but
never implemented?  Am I missing something?