What version of GLSL is equivalent to OpenGL ES 3.2 SL and do they have full parity or are there missing features? Is it safe to rely on an older version of The Orange book when learning to do ES shaders (primarily interested in Compute shaders)?
There is no version of desktop GLSL that is exactly equivalent to ESSL 3.20. There are some features of GLSL that never made it into ES at all (shader subroutines, good riddance). And there are some features that, while technically part of both, only really matter in ESSL land (precision qualifiers, for example).
If you're not using ESSL-specific learning material, the best you can do is to reference the ESSL 3.20 specification when a new concept is introduced to make sure that it's available in ESSL.