Wish it wasn't so rare to get good modding tools. From your examples, Valves commitment to their SDKs (for CS) and Warcraft's WorldEdit (DotA) are both nice ecosystems to work in.
No point in making them. Almost all big games now feature some multiplayer, if it isn't the primary focus entirely, and most games are designed from go to resist any and all modification lest people get in and ruin the fun for everyone else.
It's tremendously sad. Modding scenes in various games were huge on-ramps to development, game and otherwise, for lots of big industry figures today. Where's that coming from for the next generation's when every product is locked down from factory to where DRM regularly cripples peoples PCs?
I'm certainly no huge name in industry, but my first experience with anything even resembling code was when I discovered you could edit the scenario files in Driver (1999) on the PC to give your car all kinds of weird abilities, change how the game ran, give yourself god mode, disable time limits, all kinds of stuff, just by changing the text in the files. And like, obviously that's not software development, but I was a little kid and that was my first experience of "if you change the files inside the program, it does different things!" and that was tremendously exciting for me at the time.
The modding scene still seems pretty healthy to me? R2modman's plethora of mods and games as one example. People seem to be very willing to make ways to mod games that don't come with official support, at least from some of my more recent modding experiences. It doesn't seem like the knowledge gets socialized as much as it used to though, or it's hidden from the public web more than it was (e.g. it's in Discord groups and DMs). Those who might tinker with files still seem to get by by installing other people's mods and then messing and remixing those which is kind of cool. But for most games it's _all_ community driven.
It's still rare to get modding tools from the developer though. Only a couple recent games I've played do that I've seen. Like Teardown for example
Plenty of games still being modded, even multiplayer, have you looked? And is no problem even in multiplayer as long as server enforces which mods are enabled.
GTA 5 RP mod is big, Arma series are very bare bone games where modders created scenarios and game types. Both pubg and dayz comes from arma 2 mods.
Steam Workshop has made it easier than ever before, no need to run 3rd party launchers or visit a number of sketch websites to get your mods.
