Resident Evil Outbreak File 1 and 2 were the last "classic" style RE games that came out before RE4. They were also multiplayer-focused online co-op games... on the PS2. In 2004. I said "still playable" in the title but honestly, even for people who were RE fans back then, it's probably more like "actually playable for the first time." The fan server is also definitely not some new thing- I think it's been around for like a decade at this point.
If you've ever wondered what it would be like to play an old school survival horror game online with friends, these are the games for you. They're also pretty hard though, even if you're someone who can breeze through every other game in the franchise.
The tutorial I linked goes over through this stuff as well, but you'll need the following to play:
- An account on the fan server forums (free)
- ~~Legitimate American and Japanese copies of both games, ripped to a PC so you can run the required patchers to patch the Japanese isos into English~~ lol just kidding, just grab English-patched Japanese isos off the Internet
- Memory Card files with the required online data on them (linked in video description)
- PCSX2. As long as your machine can run PCSX2 and has an internet connection, something like a Steam Deck should work fine too (you can also apparently play the game online using an actual PS2)
You'll probably also want to wrangle some friends to play the game with you, which in my experience is the hardest part 
Having done a couple of test runs with a few different friends, I can agree that yeah, the adlib system is cool... but I definitely wouldn't want to have to rely on it. If Capcom made a game like this today (which they won't since they seem to prefer failed live services like RE:Verse or that Resistance thing) a proximity chat system like in Lethal Company would probably be a good fit.
Also of note fans brought back Monster Hunter 1,2,3 and Frontier too if you want more Capcom online slop.