I have seen that a few are doing this as a signature within post body as well.
As far as I know, yes.
It is not forbidden to include any other forum URL or a Telegram group URL in a post.
(as long as it doesn't include a referral link / code, in which case it should be reported and removed)
However, there is a distinction between :
- occasionally mentioning a service / website / URL / Telegram group;
- and spamming it all around Bitcointalk.
For more insight about the general code of conduct around Bitcointalk, you should read this sticky :
Unofficial list of (official) Bitcointalk.org rules, guidelines, FAQ