With Chrome 73, which was just released, Google enables a new feature called "Chrome Lite Pages" for Chrome users on Android phones. If you enable the Data Saver (you get prompted to do that at least on Chrome Beta on Android) the pages you visit will be a Lite version, if Chrome think they are slow (see the public announcement https://blog.chromium.org/2019/03/chrome-lite-pages-for-faster-leaner.html). More info about the feature can be found at https://www.chromestatus.com/feature/5148050062311424 (but it is still pretty vague).
When the Lite page is served, Chrome sends back the full URL the user tries to access to Google that then will get the page in a proxy and send back a "massaged" version of the page. All of this with the browser hiding the fact that the lite page is served from a Google server instead of our own.
We can disable this by setting no-transform in a cache-control HTTP header.
This seems like a privacy concern, because unlike other Chrome features like "Sync" that send all the URLs people visit to Google, it's very unclear in this case that something like this is happening. To an end-user, it might look like we are responsible for the lite version of the website (after all, the URL displayed in the browser is ours).
Overall, enabling no-transform is probably a good thing regardless of Chrome Lite, as it's a signal to all proxies that we are against any transformation (eg. injecting ads...): https://tools.ietf.org/html/rfc7234#section-220.127.116.11