Weird to see a DLC for a game that's older than Steam. Would have been more interesting if they released the WADs or whatever that you could play on the open source version of the game. (All of this is interesting in light of MS's recent hallowing of their game studios.)
When Quake was popular, CDs were far and away the most popular install method. They're locking a lot of OG owners out (unless they're hoping to drive a trickle of $4 Steam sales).
Unfortunately they break the limits of the original game engine and usually require something like Darkplaces[0] to run. Although I highly recommend darkplaces for even players of the original Quake because the QoL improvements are nice.
Also love that there is a 50% chance that you can crash a Ps4/Switch just loading a level with the Slave Zero mod that they provide. They are pushing it fairly hard with that one.