Minecraft and Terraria don't accept mods, you literally have to risk your device and data to download a third party software which rips off the actual game to get "Mods". (and when I say "rips off the game" I mean basically steals the game client, but "tweaks" it, possibly by adding Viruses and Malicious Code)
The developers of these kinds of games don't condone
ing around with their game, and potentially getting Viruses to get "new content" as cool as that content may be, and...The dev's haven't quit, nor ended the game, they're still going, they can still make as good of content as anyone.
Anyway, I will never, ever, ever, ever, ever, ever expect the developers of a game to openly support mods, to the point that there is a "Mod Menu" in the Vanilla UI.
And if a developer adds content on top of a finished game, I.E, Halloween Update for FNAF, Reign of Giants/Shipwrecked! for Don't Starve, or legit level designs for Portal 2, it's called Downloadable Content (DLC) not "Mods" (Although, Portal 2 has community-created DLC, through the Steam Workshop, I believe, I don't have a powerful enough computer to run it and find out)