In the UK these are called sluice valves, and they are very common in order to isolate parts of the network for maintenance.

Well, they are probably called sluice valves in lots of other places, but the UK specific thing is the signage.

Keep your eyes peeled for telltale SV signs (shown here alongside a fire hydrant indicator):

https://c8.alamy.com/comp/DA5TFN/fire-hydrant-water-locator-...

Probably have to terminate the fiber before the valve, put a port in to get it out, jump the valve, and then start again in the pipe after it. However once you're inside the house and about to hit the main valve they'd probably just pull the fiber out and put in the box to turn it into a network connection.

That's exactly how they would have to do it, and it wouldn't just be the fiber but a pipe to contain the fiber.

Oh, good point.

Maybe they plant to use the waste lines which shouldn't have valves on them past a certain point.

the toxic sewage that occupies most bandwidth deserves this

Then, in addition to the Internet being full of shit, the shit will also be full of Internet. A pleasing symmetry.

Some will literally have their minds in the sewer