Multiplayer games, especially MMORPGs, are notoriously hard to develop. BYOND (short for "Build Your Own Net Dream") aims to achieve the near-impossible by offering an accessible entry point for anyone to create a 2D multiplayer game... and it succeeds at it! It can be used for singleplayer games too, but multiplayer is where it truly shines.
Strengths
You heard the man, go try it!
Human-readable code: BYOND's Dream Maker (DM) tools and language aims to make it a simple process to go from zero to having a functional game. DM is one of the least intimidating languages I've come across. I can't speak to its full complexity as I've only dabbled, but it's delightfully accessible at the early stages for sure.
Active development: Here we have something of a unicorn - an indie engine over 20 years old that still maintains an engaged community and continued development! If you want to be a part of the latest and greatest, pick up a membership and join their Discord.
Includes networking + account system: You can make use of the BYOND account system to allow players to sign into your game. With this sort of tedious functionality already out of the way, you've got a lot more time and energy to spend on the important part: your game!
Weaknesses
Linux compatibility is iffy: Linux is only officially supported as a development and server platform, so you'll primarily be exporting to Windows. This really shouldn't be a dealbreaker even if you're a turbo nerd like myself. According to the forums, Wine compatability is overall solid, however.
Single maintainer: And this is the reason for the last point. I have absolutely nothing against solo projects, but I wanted to mention this so you can temper your expectations and (hopefully) go in prepared to give back in some form, whether that's with a membership or being an active community contributer. My biggest fear here is that burnout/lack of funds/an ill-timed meteor could spell doom for the project, but I'd estimate I'm overly paranoid given BYOND's lengthy tenure so far.
Great use cases
Hobbyist MMORPG/MUD creation: I spent a LOT of time as a teenager dabbling in making an MMO. It's an exercise in frustration if you're trying to make something substantial, but you can certainly learn a lot from trying to make one anyways. BYOND is excellent for exactly that sort of curious experimentation.
Noteworthy Projects
There's 20+ years of games, y'all. I've barely scratched the surface. I'll update this later when I've had a chance to poke around more and pick out a few favorites. In the meantime, if you come across any that you think deserve a shoutout, let me know! I want to spotlight any gems, especially if they're active multiplayer projects. :)
Learning resources
Developer resources list - You can find tons of great tutorials across the forums, but this is a particularly helpful starting point.
Other
If you give BYOND a try and enjoy it, you should definitely toss the devs a couple bucks if you can spare 'em. Projects like this tend to be horribly underfunded and are sustained by years of passion from maintainers. I have no clue what their server costs are, but that stuff adds up faster than you'd think!