Does Apple Watch Series 6 GPS+cellular support 5G SIM?

It uses an eSIM and it is LTE.

The Apple Watch shares the iPhone's phone number, and as such the carrier must support the Apple Watch, and will charge a monthly fee for the phone number sharing. In the U.S. the fee is $10 per month added to the iPhone bill.

Here is a list of carriers that support the Apple Watch

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

Please clarify whether LTE is limited to 4G or whether it also includes 5G support based on the region/market. To be more specific, T-Mobile/Sprint (US) uses Band 71, but band 71 is not listed in the above-referenced URL. T-Mo's 5G network is still be built out just like other carriers.

Ultimately, is it expected that the eSIM in the Series 6 is upgradeable to include the expanded 5G network in the future? Thanks in advance.

4G level LTE. The 5G chips suck so much power the Apple Watch would melt it is tried to use a 5G chips today. The 5G tech has a ways to go before it ready for something like the Apple Watch.