6 person parties has a lot of history, but there's some fluctuation
Gold Box games - 6 party members, possibly 2 NPC companions for some segments Infinity engine games (Baldur's Gate and Icewind Dale) - 6 party members Eye of the Beholder - create up to 4 and supplement with companions up to a max of 6
Pillars of Eternity, not D&D but clearly inspired, also I believe has up to 6 party members active.
For other variations:
Demonstone had 3 party members (though it was an action game with set characters)
Neverwinter Nights 1 had 1 PC with 1 companion, possibly 1 summon for each companion depending on spells. Neverwinter Nights 1: Hordes of the Underdark allowed up to 2 companions
Neverwinter Nights 2 had 3 NPCs in Act 1 and 4 NPCs in Acts 2-3 for a total of 5 party members
NWN 1 and NWN 2 are the only non-6 franchises that have similar level of fame and nostalgia going for them as the GBG, Infinity, and EotB games.
Sword Coast Legends limited you to a party of 4
Ravenloft games (Strahd's Possession and Stone Prophet) allowed you to create up to 2 and recruit up to 4
Menzoberranzan I'm uncertain of, but you make 4 characters
The recent Dark Alliance is 4 characters, but that's set characters again.
I'm uncertain of the party size limits in DDO and Neverwinter
Art from AD&D and 2nd ed implied party sizes from 4-6 and modules were written for parties of 4-6 members
Novels imply party sizes of about 3-5
Knights of the Dinner Table and similar community comics tend to show parties around 3-5 members in size, probably due to the added difficulty of scripting extra characters.
Critical Role generally runs around 7 players, but all of them are professional actors so their day-job dovetails with smooth gameplay
A lot of streams run 4 - 5 party members.
3rd edition seemed to also assume 4-6 party size. 4th ed seemed to assume 3-5 party size.
While there's no explicit party sizes in 5e adventures, the art seems to suggest 4-5 party members, again that may just be for reduced art complexity.
Design space for RPGs in general I've been seeing a lot more aiming at 4 players as average and even a lot of games geared toward sizes of 2 to 4. (If you play D&D with 2 players, I suggest the method my brother took of having a system of hiring companions and the players would control some of their friendly mercs in combat and do some light RP for said mercs to keep the burden a bit off the GM...)
I suspect think the general increase of the average age of the gaming hobby has come with an increased difficulty of scheduling 7 people to a unified schedule, hence the shift towards 2-5 group sizes. Some older games with heavily specialized roles like Shadowrun (Mage/Decker/Samurai/Face) and D&D (Mage/Cleric/Fighter/Rogue) have been balancing more towards 4 than 5 or 6.
This being a CRPG, a 6 person party isn't going to stress out and burnout a GM (it's a computer) which is a concern on TTRPG, but the trend of computer games is running toward four-player as well (Phasmophobia, Dark Alliance, Deep Rock Galactic, Borderlands) Though with this is certainly not universal.
I am personally a fan of 4-person max for personal health purposes when running a game but in a computer game I'd like to see a stretch to more characters.
