This would only be possible if you were talking from a jumbo-configured client (let's say you've set up your laptop correctly), across a jumbo-configured network (Starlink, in your scenario), to a jumbo-configured server (here's the problem).

The problem is that Starlink only controls the steps from your router to "the internet". If you're trying to talk to spacex.com it'd be possible, but if you're trying to talk to google.com then now you need Starlink to be peering with ISPs that have jumbo frames, and they need to peer with ISPs with jumbo frames, etc etc and then also google's servers need to support jumbo frames.

Basically, the problem is that Starlink is not actually end to end, if you're trying to reach arbitrary servers on the internet. It just connects you to the rest of the internet, and you're back to where you started.

This is also true for any other ISP, Starlink is not special in this regard.

Well, depending on the quality of the connections, a re-transmit of a jumbo frame could mean having to re-transmit a lot more data.

But since the local network and the end network where the servers are located will almost certainly be 1500, the point is almost all but moot.

Most connections will not be peer-to-peer over Starlink, so you need to deal with the least common denominator.

Because you don't know what you're talking about and are engaging in "what if"-isms? There is no business case to solve with jumbos frames over the Internet. I've been in this business for 20 years. Seen this argument a dozen times. It never changes.