When using Golurk V's "Rewind Beam" attack against an evolved Pokemon, does my opponent get to put the highest Stage Evolution card into their hand if Golurk V does enough damage to KO it?
All effects of attacks must be completed before checking for KO's. So yes, the highest Stage Evolution card is put into their hand before it is KO'd.
Source: TPCi Rules Team (2023-11-16)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH7_70
SWSH7_70