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...

This event sounds much more realistic/common, the motivation of an ISP to save bandwidth costs is much more likely/frequent than the motivation of an ISP to monetize through ads (in addition to monthly service fees).