It's not like the mod scene is dead. The big game mods have migrated to indie development through Unity - if somebody made Counter-Strike today, it would be a Unity game because the tools are so easy to use and productive compared to the old Hammer toolkit. And as for modding in general, you just need to look for the right games, where it's still thriving. Minecraft, FTL, Sins of a Solar Empire, Stellaris (and other Paradox games), Elder Scrolls all have varying degrees of active communities (some have aged less poorly, others are still alive and kicking - but all of them have fantastic mods that make it worth playing these games over and over). Even SoaSE, whose population has really dwindled, still has a handful of total conversion mods that are still in active development (one of which is arguably one of the best Star Trek mods out there for any game).

The industry has definitely changed, but I don't feel like there are less mods or less people interested in making mods than there used to be.

The latter-day Half-Life and early-to-mid Source engine mods... such a good time. People were cranking out all kinds of crazy stuff, pushing those engines to their limits and then some. Garry's Mod kept this going for awhile too, with people making crazier and crazier custom gamemodes and maps... man I miss those days, there was always cool new stuff to discover, communally online with others.

I'd recommend checking out any game developed by Paradox. You haven't seen fantastic mods until you've seen what people can do with Paradox games

For that matter, some games that Paradox only publishes (e.g. Cities: Skylines) also have fantastic modding support, though that's hit-and-miss compared to the games they develop in house.

I love Paradox. I happily fork over $10-20 a few times a year for new EU4 and Stellaris DLC. I feel like (1) the longevity of those games, (2) the ability for the developer to majorly rework systems far past release (see EU4 Art of War), and (3) modding ability, are all related.