Will Apple Watch series 9 work with a prepaid SIM?

if you scroll to the bottom of the page and click united states if you don't live there and choose your country

and then click as if you attempt to purchase an apple watch it will list the carriers that support the watch in your country and then see.

or you can ask your carrier

Here is the list of carriers that support the Apple Watch

https://www.apple.com/watch/cellular

Generally prepaid eSIM’s are offered by MVNO (Mobile Virtual Network Operators) which have a contract to resell services of a major carrier. Most (but not all) MVNO contracts DO NOT include Apple Watch support. There are exceptions, such as in the U.S. Xfinity and Spectrum are MVNO’s which support the Apple Watch.

If your MVNO carrier is not explicitly listed in the above list of carriers, then they do not support the Apple Watch.

NOTE: A GPS only Apple Watch, or an un activated cellular model, can do EVERYTHING an activated cellular model can do, AS LONG AS the paired iPhone is nearby.

I have been using a GPS only Apple Watch for 5 years, and because I always take my iPhone with me, the Apple Watch has been able to make and receive phone call while riding my bike, get weather, send/receive text messages, set reminders, etc….