A few times over the years Ubuntu included Amazon ads in the OS. Each time, afaik, the community reacted angrily and it didn't last.

Once ever. The default search returned results from Amazon and local files potentially leaking your search intended to find local files to Ubuntu who in turn claimed that it was ok because potentially intensely personal info that could be inferred from queries weren't personally attributable to you.

This was obviously not ok and it never happened again this was if I recall correctly around 2012.

Ah thanks. I did a quick search before posting and this article was listed as from 2019, but that was when it was last updated - it did just happen once in 2012.

https://www.omgubuntu.co.uk/2012/09/mark-shuttleworth-explai...

There's also a couple places where Ubuntu advertises their commercial services in the OS, including in apt ("Get more security updates through Ubuntu Pro...") and in the default login message (promotions for Ubuntu Landscape, as well as various other products and services through motd-news).