You mean the trigger/action system in StarEdit? I wouldn't call it a programming language, but it was a very nice introduction to scripting, especially for me as a 10yro kid at the time. I probably owe a bit of my career to it.
Obviously fair share of problems, but there were some clever workarounds with "hyper triggers" to make triggers fire very often, instead of being recalculated every 2 (?) seconds... or stuff like counting minerals to do things (what to be honest was horrible, but worked).
Those custom maps grew as our computers grew (I cant imagine some of the custom maps working with 16mb of RAM, luckily I upgraded to I think 192) and spawned whole generes - MOBAs and Turret defense. There was even that "the unknown" map what is basically Among Us long before Among us.
Are there any games nowadays that allow such things and spawn so many custom maps?
WC3 literally spawned DotA, and thus the entire genre of MOBA. It was originally just a WC3 map.
SC2's editor is also quite capable - the game is free to play, there is a lot of cool stuff in the arcade section, with some very active minigame communities (Direct Strike, Crap Patrol, etc). Sunspear Games are making Immortal, a new RTS which was initially prototyped using the SC2 engine; these same guys earlier did StarBow, which could best be described as "StarCraft 1.5", with many good elements of both BW and SC2. And of course there's that guy who's remaking all of WC3 in the SC2 engine, and it works better than WC3 Reforged (which I sincerely do not recommend to spend your money on). There's also the SC2 Mapster Discord server, some people are trying quite interesting things for regular 1v1 maps (considering how conservative normal 1v1 is), like water slides, destructible resources/watchtowers, etc.
Overall not the golden age, but it's very much alive.
(Or there was a map with same idea before - in SC:BW).