After some Googling it seems most discussions of the evolution of command-line arguments couple the rise of the = sign to the rise of GNU-style long options. I can't find a reference stating explicitly that it was never common practice to use = to separate short options from their arguments, but I also can't find any discussion of anyone doing that.
Some brief discussions ...
- Art of Command Line Programming - Command Line Options - see introduction
- perl Getopt::Long - see "Command Line Options, an Introduction" section
- Command-line interface - no discussion of history, but perhaps further clicking might unearth some
