I think I get what
@tetrasodium is saying. Basically, the DM/GM would be able to hand out and sprinkle those additional weapons, powers, and abilities across their game as suited the DM/Game. Thereby allowing the DM to sculpt the game.
5e has taken that "ability" from the DM, for the most part, and plopped it right down in the players laps. And, if you read a lot of what gets posted on this and similar boards about "player agency", then no, the DM really isn't viewed by many (or a vocal minority), as the "primary resolution mechanic" because it, by definition, interferes with player agency. (I'm sure my english teacher would flunk me for that sentence, good thing I graduated decades ago

)
That being said, in our group, which has been meeting and playing together for about 40 years or so, we still ascribe to the "GM says" school of DnD RPG, and aside from the usual "rules lawyering" (I'm the primary culprit, but am really working on it!), we let the DM decide what happens, or what can and can't be used, what items or abilities are in the game or not, etc. We've even gone so far as to start a Basic DnD campaign, and an Ad&d 2e campaign. They've been eye-opening, to say the least.
Edit: AND ninja'd by
@Helldritch