My notes are pretty scattered.

Here's some open source BBSs I had bookmarked:

* Oblivion 2 XRM => Obv 2 remake, C++, looks OK but no killer new features really and uses Boost - https://github.com/M-griffin/Oblivion2-XRM

* Enigma 1/2 => Looks interesting and possibly the best of the bunch, but also more of the same. Good ANSI support. node js-based, so major downvotes from me here - https://github.com/NuSkooler/enigma-bbs

* x84 => Python SSH/Telnet/BBS Core => Interesting, but I didn't like the code on code review, and I think both C++ and node js are better choices. Forked a few times too, so need to see what's up - https://github.com/jquast/x84

* Synchronet => I don't know much about the modern version of this, but I hated the old one. It looks quite dated, but is updated somewhat it seem - http://www.synchro.net/

* WWIV => I wonder why this cockroach keeps living, sorry. It's written in C++ and the new version may be good, I don't know since I didn't try it. What I do know is at one point I co-sysop'd someone's WWIV board and learned how bad it really is. Truly one of the worst of the popular BBS softwares ever IMO. Perhaps a base for a derivative work though - https://github.com/wwivbbs/wwiv

Some BBSs that I am relatively, but not 100% sure there is source floating around for:

* WWIV - There are 2 sources of the old, 1 Pascal and 1 C++ . The core is not so bad for the time, it's just the implementation further up is garbage.

* Oblivion/2 - I think 2.3 or something like that. Source is pretty awful, but it looked good out of the box and had lots of add-ons from art scene especially.

* Renegade - I believe this was leaked at some point, maybe official release later? It was backdoored as well for some versions. Based on Telegard.

* Forum - Used to build lots of other software. Lookup "Forum Hacks" for a ton of other source. This includes Vision, Vision/2, Vision/X. All are insanely hackable and/or backdoored. There's also Oblivion/2, Emulex, Emulex-2, and Celerity based on this I believe, and many more.

* Telegard - Based on WWIV but with monster spaghetti of Pascal, C++, and ASM. No real reason to study this unless you want to create your own hack on it, which I don't recommend. Bunch of hacks on this as well, like Renegade.

* PCBoard - Not good source (a mess), but was the creme-de-la-creme of non-underground boards and for warez boards (customized). Supported its own Pascal-like language (PPL). I think version 15 source was leaked, or at least I somehow had it/have it (somewhere). Also very extensible compared to others.

* AmiExpress - I believe source is floating around for this. Was great on Amiga and oft-copied in art scene/pcboard setups.

Other:

* BBS Archive - Bunch of source here - http://archives.thebbs.org/ra103c.htm

* Old BBS Software List - http://software.bbsdocumentary.com/

* Mystic BBS - I ran this for a bit to mess around for a few months long ago. It was never the best of the best, but always pretty good if you want something like Obv/2 but better. It seems like it's been updated a lot since and still going. Didn't find source and license is marked as some sort of proprietary freeware. Supports SSH, telnet, etc - http://www.mysticbbs.com/

In general, there's also a lot of source floating around for C64, Amiga, Atari ST, and CP/M and it's even more nostalgic than the rest. I've seen the source of tons of BBSs and they are notoriously a mess. The most common languages you'll encounter are Pascal/Turbo Pascal, C, C++, and assembler (per platform).

I'm falling asleep, but plenty more floating around. Hopefully that is a start. A few final tips

* If you want to run door games, be careful, lots of infected binaries.

* If you are running old stuff and get a hacked version of Doorway itself to run door games or a fossil driver because your BBS needs it, these also are commonly infected. Funny enough I once wasted several days trying to get a copy of Doorway to work on a friend's board, not realizing he pirated it and it was specifically infected with the Taipan virus. This copy seems to be everywhere, so clean it and it will work or find a different copy.

* Do not run any Forum or Telegard hacks to the outside world.

* If you want to build a nice BBS that's via the Internet, start with the socket side of things and don't build on a rotten core. You can do a lot with what's available on the JVM (Java, Clojure), Go, Rust, C++, Erlang, Lisp, and any language that supports concurrency to a decent amount. Don't use PHP, Python, Ruby, etc. because you're just handicapping your BBS from the start.

* If you really want to emulate the experience, you need to get ANSI support correct, and support the full spectrum of codes, fonts, and so on. Get a handle on code pages and differences between platforms like DOS vs Amiga. Some of the links above cover this pretty well.