I thought that was a one time thing in a 3rd world country blown out of proportion into myth status.

Would you mind sharing what ISP it was and what time period this was in?

I’m not sure whether this applies globally, but in Japan, around 2015, some mobile carriers deployed a “traffic optimization” feature that would lossily compress images in transit.

On the platforms of NTT Docomo and KDDI (au), users could opt out of this behavior. However, with SoftBank, it could not be disabled, which led to controversy.

As you might expect, this caused issues—since the image data was modified, the hash values changed. As a result, some game apps detected downloaded image files as corrupted and failed to load them properly.

Needless to say, this was effectively a man-in-the-middle attack, so it did not work over HTTPS.

Within a couple of years, the feature seems to have been quietly discontinued.

There were also concerns that this might violate the secrecy of communications, but at least the government authorities responsible for telecommunications did not take any concrete action against it.

There is a Japanese Wikipedia article about this: https://ja.wikipedia.org/wiki/%E9%80%9A%E4%BF%A1%E3%81%AE%E6...

Where as my ISP did not put in ads, they did inject messages such as maintenance was going to occur and did things like redirect bad dns to their own search.

Also ISPs were monitoring and selling browsing data years ago.

Comcast / Xfinity in the U.S., for example:

https://www.reddit.com/r/technology/comments/9b5ikd/comcastx...

Cox Communications used to do it in California to inject JS into sites. I remember seeing little Cox popup/toast messages in the corner of other sites.

it was some mobile ISP in Russia. Maybe 6 or 8 years ago