my question remains, I don't see a reason why it's not worth spending $15 or $20 to implement 2fa on this current forum
It's probably just that little bit of money, plus a LOT OF PROGRAMMING TIME.
Theymos has modded this forum considerably. That's why it's such an old version of SMF - he can't upgrade it anymore.
A "simple" thing like 2FA could break a dozen mods he has made....
that makes sense if that's the case. but has that been tested?
adding 2fa would be very important. it's definitely worth trying the plugin and checking if it really breaks something. there must be a dev/test installation of the forum I guess