I'll qualify that. Legal to listen, not legal talk on. Check out the link.
I did check out the link, including when that discussion was new, 8 years ago. The initial posting in that thread ask the question if old 23 cahnnel CBs are legal to use, it does not say they are illegal. No place in that thread does it say 23 channel CBs are illegal to use, further, no place in the thread does it quote a regulation as to why it would be illegal to use. It does quote the regulation that establishes that old 23 channel radios, made and certificated before a specific date, cannot be manufactured or marketed.
There was a "rumor" around at the time (1976/77) the regulation changed, that old radios would no longer be legal. It was persistent. But no one could ever point to an actual regulation that stated so. That rumor has lingered to today and is still around, but people are still unable to provide anything verifiable that makes 23 channel radios illegal to use.
Certainly, a radio built to the old spec would not be legal to make today. It is not the fact that the number of channels changed, you can make a CB with any subset of the legal channels you want. But several other technical factors changed that the original CBs would no longer meet. Frequency stability, 4 W output power vs 5 W input power, maximum PA dissipation, etc. But the FCC, back in the day, clearly said that continuing to use the old radios would not be considered a violation.