Richard Clark, KB7QHC wrote:
"The velocity of an imaginary point contains no information nor energy."

You could imagine a point like that. I regret omitting the word
"imaginary". It was unintentional.

I gave a dictionary definition of "phase velocity". It seems to be
synonymous with propagation velocity. The same dictionary defines group
velocity, and says:
"Group velocity differs from phase velocity in a medium in which the
phase velocity varies with frequency."

If phase variation with frequency is severe, and if the signal contains
multiple frequencies, I think it would likely alter the waveform.

Best regards, Richard Harrison, KB5WZI