There's always Don't Starve which definitely fits the mood, though I've always found that game insanely difficult.

Anything by Chilla’s Art. The Coffee Shop, Parasocial, The Bathhouse, Shinkansen 0, The Radio Station.

Also The Hex if you want a spooky mood but no jumpscares. Go in blind, it’s by Daniel Mullins who did Inscryption. It’s an incredible game. Ignore the screenshots on Steam and don’t watch the trailer, it’s five USD and the graphics make sense after playing for 30m.

If you want jump scares, At Dead of Night. My partner and I are horror aficionados and we cannot play that game.

For spooks, Phasmophobia is a good one. They do a Halloween event every year too 🎃👻

Inscryption, 100%. Such a good game. Tons of post game content, too.

Pumpkin Jack. It's a 3D platformer. I haven't played it in a couple years, but I remember it being mostly linear. Not a ton of collectables, but some. 11 months out of the year it's a pretty "meh" game, but it absolutely NAILS the Halloween aesthetic. Not "horror" or "scary" or "autumn" but very specifically Halloween.

MediEvil is similar, though much older. I have only played the original for PS1, though there is a modern remake on all platforms that looks pretty good. Not quite as explicitly Halloween-y, but still pretty close. Flawed in its own ways, but I would still say a better game overall than Pumpkin Jack. The levels were a bit less linear and it was a bit more like an adventure game than a platformer.

Luigi's Mansion is a classic too.

A lot of other games have levels or worlds that are good for Halloween even if the whole game isn't. Like Pumpkin Hill in Sonic Adventure 2, or Subcon Forest from A Hat In Time. Honestly one day I want to compile a list of all of these themes areas across my favorite games and the play all of these levels seasonally.

If you'd like a less serious, funny comedy-horror game, I enjoyed The Upturned.

Some less silly games I'd also reccomend are 10 Dead Doves
and
Withering Rooms

Amnesia: the dark descent

The secret of Monkey island games have a tropical Halloween feel with ghost pirates and root beer

Kentucky route zero for a slower ghost story, dear Esther for a walking sim ghost story both with great soundtracks

I remember they used to do gm run player events around Halloween in EverQuest back in the day and on project 1999, not sure if any mmo or online games theses days still have human gms

Eternal Darkness for Gamecube.

I can't invest the time in MMOs anymore but I always miss them around this time of year. Guild Wars/Guild Wars 2 did a great job of it.

Costume Quest is great for a single player game.

I'll also throw out a classic - Ghost Master. You manage a team of ghosts haunting different locations.

How about Jack in the dark, the little game that shipped with the cd version of the two first Alone in the dark games?

SOMA is one of my all time faves.
The Outlast Games are also classics for Halloween

Vampire Savior

The 7th Guest if you want to play a puzzle game with campy acting

also Deus Ex Randomizer has a Halloween mode

The Cursed Forest is fun.

You already mentioned the two games I know of that do halloween events, so here's just a list of spooky horror games that are extra fun to play in October:

I liked Indigo Park. But, like... Not because the gameplay. I just really like Rambley.

But Resident Evil (any other than Outbreak) is spooky.

Amnesia, Outlast, the old Hugo point and click adventure games for DOS, Phantasmagoria, Phasmophobia, Slaughterhouse, Castlevania...

Killing floor used to do good holiday events, but it's been years since I've participated, they may not anymore!

Phasmaphonia is pretty cool, multiplayer co-op with voice chat. Basically you play as a team of professional ghost hunters who have to investigate haunted places. Can be scary AF but awesome atmosphere and cool culture.

Overwatch had a PVE horde defense thing that came out around halloween that was fun as hell. I don't know if they still do in the new one but that was the only reason I played that game.

Sounds like you're focused on multiplayer games, but the Echoes of the Eye expansion pack for Outer Wilds is a phonemonal "spooky" and eerie game without being a true horror game. Highly recommend both it and the base game.

For board games, Betrayal at House on the Hill is a good one! Maybe also the Fury of Dracula board game.

Cryptmaster! Not specifically “Halloween” but undead and spooky and witty. Great if you like puzzle games and dungeon crawlers.

Look Outside just had a major 2.0 patch. Solid horror game. And it’s 30% off at the moment.

Steam link

Ghost Town Pumpkin Festival carve a pumpkin, explore a haunted house, other Halloween things, all in a multiplayer environment as little ghosts

Not particularly a whole game but the halloween day/nite in bully. It always gave me the perfect Halloween vibes.

Deep Rock Galactic if you don't want anything too spooky. Other than that, my favorite horror game is Until Dawn. I still have to try the Quarry.