I don't recall it being passed as law....

However, several years ago there were some FCC rules that locked out cell phone frequencies from access by scanners.

So, your observation of older scanners having more freq coverage is probably correct.

While some radios have gone up, others have come down. It is now more affordable that ever to move into 2 meter.

Depending on the taxi radios, you may come up on an previously allocated frequency. That would be bad news if you tied to use them.

And, yes it would be illegal to use a radio in a requency not allocated for that use.