Some ‘getting started’ suggestions:
Telnet BBS Guide has over a thousand BBSes listed, most are accessible by telnet. Syncterm is a great terminal program for BBS use, with Linux/Mac/Windows versions available. Other telnet clients can be used, but many BBSes use ANSI and CP437 and not UTF8. (other BBSes use other standards like ATASCII or PETSCII, Syncterm supports many of these.)
There are a bunch of interesting BBSes, one I’d recommend is 20 For Beers, connect to it at 20forbeers.com:1337. Plenty of great ANSI art and active message areas and a huge file collection too. I also recommend checking out the fsxNet message areas, they are shared across many BBSes and have an active community.
Speaking of Contra - there is a new Contra out March 12, 2024 (today! or tomorrow depending on timezone), Contra: Operation Galuga. I haven’t played the game yet, but the trailer looks amazing!