It depends on whether the station continued to air the same decades or not. There are stations focusing on the 90s playing "classic hits", are there not?
A station does not have to include the 90s to be called Classic Hits, but very few (if any) still play the 60s and the number of 70s songs that fit most stations' demographic composition is dwindling as well.
If the category includes stations playing the 90s.
You don't understand any of this, do you? Yet you persist on creating a definition of the format that is contrary to the industry's.
Here is an example of the relative importance by decades: KRTH, one of the best known stations that transitioned from Oldies to Classic Hits.
Their typical playlist is around 300 songs. 75 are from the '90s, another 16 are post-2000. 25 are from the '70s, and four of those are also in my all-80s format on KRKE because they are songs that listeners
perceive as being '80s titles. Total spins of pre-1980 songs, 251. Total of songs from 1990 or newer: 318. Everything else: 1,296.
For comparison: KOSF, which is well-known for having expanded beyond its original all-80s concept (or so they try to convince their audience). 460 songs, 53 of which are newer than 1990 but comprise only 197 spins out of a total 1,996. 70's? 98 songs, 248 spins, and most only get a token one spin per week.
I can also show you stations that are a mix of 70s/80s but cut off somewhere around 1986-1987, and they are also called Classic Hits.
So ...
NO, 90s is
NOT a key make-or-break factor in what they are called.
Now stop it. Your definitions do not have any correlation to what the accepted industry ones are. Period.