It's been 3-4 players for me for 37 years ...
I went from routine 5-9 down to 3-7.
More importantly, advised and designed for group sizes have shrunk.
Most games now have sweet spots in the 3-5 range, while certain in the past worked best with 5-7.. part of the resistance to D&D 4E, for example, was it was five core roles, instead of the "D&D standard 4"... as in the 4E era, most groups in homes were 3-5 players.
Still, while 3-4 is playable in many games from the past, it's always been suboptimal in several. AD&D1 you want 2 combattants (Fighter, Ranger, Paladin, Cavalier) a thief, a cleric, and a wizard; at low levels, add another cleric or a druid, and add another mage or an illusionist. Monks and assassins could kind of supplant the thief... but were better as add-ons.
Gygax and Arneson both would not infrequently have 10+ players show
and play...
Meanwhile, L5R has 4 PC roles: Bushi, Shugenja, Courtier, and Monk... Monks have always been "outside the balance"; L5R 5 has made it such that Courtiers and Bushi can cross-train easily and effectively... a 2 player party can handle most storylines.