I think akvadrako above[1] has it right, they appeared on my mobile phone on sync, as they are bookmarks.

Again, I think, I'm not sure ;)

[1] https://news.ycombinator.com/item?id=22115461

There are two things being confused here.

I was thinking about OpenSearch custom search engines (being deprecated apparently as another comment notes, these appear on desktop as icons under the search input field "This time search with" and similarly in Android as icons above the on-screen keyboard. These definitely don't sync.

The other thing are bookmarks with the "keyword" property set. These do indeed sync, like other bookmarks.