This tweet lead me to researching this topic, so there are at least two persons witnessing ads on itch: https://twitter.com/ryguygames/status/1330541234876805123
1) use browser thats not logged into itch.io
2) ensure browser has no adblock installed / tracking protection enabled
3) go to https://faxdoc.itch.io/kattespill
4) click on download now
5) click on No thanks, ...
6) click on download
7) Wait a second for the ad to load on the post download pop up