I recently got a '92 Legacy SS 4wd Automatic 4wd that needs tires. Having owned later model Legacys '96, '97, '98 04, I'm aware of the recommendation to replace all tires at once and ensure that the diameter is within 1/4", or whatever the manual says, for the longevity of the 4EAT auto transmission.

Does this apply to my '92 SS as well, or is the auto transmission in that a little less picky about tire diameter?

Common law on subaru awd still applies.

Common law on subaru awd still applies.

Agree, since  the 92 is AWD, it still needs all 4 tires to match.

It's a subaru AWD requirement. It will create premature wear and can cause major damage to the differentials.