FLStudio 11 project in FLStudio 20 will work but not FLStudio 20 in 11, or even FLStudio 20.1 in 20.0.
Edition considerations (Fruity, Producer, Signature etc) - While you can open a higher edition project ...
FLStudio version, in an older program?
Answer
It's not possible
Extra information
It has always been the case projects from the latest version of FLStudio do not load in older versions as many ...