Just near me, a Roman road about 5km long was discovered last year by exactly this method. That's a non-trivial discovery. There was also a large Roman villa complex discovered the same way.
For LIDAR and structures, you're likely correct - there's vanishingly obvious structures left to discover that way.
https://www.bbc.co.uk/news/uk-wales-52911797
"Roman forts, roads, military camps and villas have been identified by a new analysis of aerial photographs taken in the 2018 heatwave across Wales.
Scorched crop marks uncovered about 200 ancient sites during the drought.
Experts say the Roman finds are key pieces in the jigsaw to understand how Wales was conquered and dominated 2,000 years ago.
Researcher Toby Driver said the discoveries "turn everything we know about the Romans on its head"."
Those pictures would be very appropriate for AI bots to look at and make discoveries.
Although it does feel the finds are getting increasingly hard to spot, every now and then you get a _how did anyone miss that_ moment. Perhaps not surprisingly, most of the remaining impressive finds tend to be in woodland, using the Digital Terrain Model data which can peer beneath the forest canopy, like this Mexican one today, or are within newly released coverage which is promptly pounced on by armchair archeologists.
Not sure what my point is here except... to me this story still feels like a function of the amazing recent progress in lidar coverage and data availability (not to mention the incredible technology behind it), rather than an AI one, although I don't doubt that pointing a capable model and a healthy token budget in this direction might well be able to sweep up a few of the remaining discoveries.
Also agents analyzing drought pictures, where many ancient UK structures seem to appear would be an easy win. I've never checked whether there's a comprehensive drought picture map of the UK.