I'm a bit surprised by your premise, games with local co-op have certainly almost disappeared entirely, but I haven't noticed a trend in online co-op. Some good ones off-hand (that haven't been mentioned). These are in descending order if my personal playtime as a rough proxy for how good I think they are.
- Remnant
- Wasteland 3
- Outriders
- Gloomhaven
- Risk of Rain 2
- Warhammer Vermintide 2
- Monster Hunter World
- Left 4 Dead series
- Back 4 Blood
- Borderlands series
- Magicka
- Star Wars the Old Republic
- Helldivers
- Barony
- Barotrauma
- Gods Trigger
- Action Squad
- Aragami 1,2
- Evil West