An NPC from sw/sh mentions that some gyms are moved to the "minor league," and I remember it coming up in some translated interviews as well. what is it?
There are 18 types of Pokémon, each with there own gyms. However, only 8 are in the major league (the same 8 gyms you battle in game)
The rest of the types are in the minor league which you cannot battle with the exception of the Ghost and Ice type gyms since in the Shield version those minor league gyms are now major league gym from the Sword version.
Here is a link with from an interview with a representative of The Pokémon Company International to back up my answer