Maybe that wasn't possible/enabled for our particular deployment (that was in the IT dept. of a big European bank), as we had what I've been told was a strange bastardization of O365 and other MSFT services - a mix of on-prem and cloud services unlikely to be found elsewhere.

At any rate, for the IRC server at the shop I was at previously, I had "webhook" support/a http(s)->irc gateway implemented on my own in much less than an hour via CGI ;)

Isn't your comment then very specifiy for that installation of yours? When you use MS Teams in a "normal" O365 environment, then every feature you described for IRC is possible with weebhooks and the cards API. Simple CURL based integration.

I will second with another anecdote at mega corporation - Teams is entirely locked down and any of the useful integration points are disabled.

Same with slack webhooks. I make custom software and everyone wants slack webhooks and they are pretty easy to implement and people love them.