I don't know if it's true, but I've heard that some ISPs route your entire traffic through their machines. They even have access to your IP packets. Very shady!
ISP's intercepting HTTP traffic to modify it is far from unheard of.
In the best case, this is to notify customers of required changes.
This is actually used by comcast [1].
In the worst case, this is a service sold to advertisers, or a service that includes arbitrary java-script injection. For something close to the worst case, see [2] (previously discussed on HN [3]).
[1] https://tools.ietf.org/html/rfc6108
[2] https://defplex.wordpress.com/2017/08/15/how-a-south-african...
[3] https://news.ycombinator.com/item?id=15423393