Sheldon Brown wrote:

Since the biggest and smallest cogs aren't any farther apart on a
ten-speed cassette, what is the obstacle to using a nine-speed rear
derailleur with a ten-speed shifter, cassette, and chain? Is it the
pulleys?

No, it's the marketing.

Sheldon "They Want Your Money" Brown

Is that the reason, seriously? Or is it because they genuinely want their
customers to have the best and are terribly consevative?

2001+ 9-speed rear derailleurs are virtually the same as the 10-speed
versions (geometry and ratio is the same, AFAIK). Effective width of 10sp
pulley wheels is slightly narrower (different spacers). From memory,
difference is 0.2mm or 0.4mm.

~PB