Only Ritter vi is really a fork. All of the rest are clones, ground-up reimplementations not forked from the Joy+Horton code.
Of them, and discounting the extreme fringes like vi-likes that run on the BBC Micro, only STEVIE lacks :x, and its derivatives all added it in. All of the other clones from MKS onwards, even neatvi and nextvi, have :x .