This was always the case in the web version of Outlook, and the mobile client. If you subscribe to 365, ads are removed.
I noticed this last year when I moved away from 365 and started getting ads on the Outlook mobile client. I ended up binning it off and just used the default Mail app on iOS, which is a shame because the Outlook app on phones is actually superb. Not good enough to put up with ads though.
So protonmail is pretty cool.
Made the switch last year from Gmail and I love it. I set up a custom domain and a catch all address as well, so I can give out whateveriwant@mydomain.com as an email and it gets to me. Particularly nice for finding out who’s sharing my information, as well as picking out phishing emails. Banking coming in from amazon@mydomain.com? Immediate red flag.