You may see an instruction in ATIS stating that the airport is not accepting “light aircraft”. Per Tyler Shelton, ATC Community Manager for Infinite Flight, here is the definition of a light aircraft.
Light aircraft include the XCub, C172, SR22, Spitfire, and the P38. The TBM930 and C208 may be used without restriction in a commercial environment.
The encouraged restriction is to separate aircraft which naturally do not mix well based on performance capabilities and characteristics. Light aircraft should go to appropriate airports that can accommodate them without undue delay to everyone else.
Airliners and aircraft like the XCub don’t mix that well.