I agree wholeheartedly with [Google's Page Annotations](https://support.google.com/websearch/thread/308719098/page-annotation-in-google-app-browser-for-ios?hl=en) being an absolutely *awful* antifeature, and recommend that others opt-out and/or protest the feature. I want to make a clarification that doesn't invalidate your main points:
Clicking annotations doesn't navigate away from your site to a Google search; it triggers an overlay with infoboxes about the term you selected. It's similar to the iOS "Look Up" option for selected text. It's wrong to do because this obfuscates what is and isn't a link the author placed on the page. Inserting what appears to be links into the page crosses the line from user-agent interventions, such as adblocking or turning off certain unsafe features (acceptable) to editing an author's words in a way that isn't required for people to read them (unacceptable).
Editing page contents is fine if it's necessary for people to read them, e.g. translations or the [WAI-Adapt standards](https://www.w3.org/WAI/adapt/). Both ideally inform the user that the page has been modified. Page Annotations go well beyond that.
