iOS 18, and macOS 15 have included a feature in Safari called Highlights.
Below is a description from Apple on how it works.
Highlights
If you enable Highlights, Safari will automatically detect relevant information on a web page and highlight it as you browse. When visiting a web page, Safari may send information calculated from the web page address to Apple over OHTTP to determine if relevant highlights are available. When you select the page menu icon to see relevant highlights for a page, Safari may send the web page’s address to Apple in order to provide the highlights. All information sent to Apple is not associated with your Apple Account, email address or other data Apple may have from your use of other Apple services. You can enable Highlights by going to Settings > Apps > Safari > Highlights, then tapping to turn on Highlights.
Due to information from your current browsing session being sent to Apple, the recommendation would be to leave it disabled as is by default at this point in time.