KMail isn’t available on flathub, so the only way you would see it in discover is if you have done what everyone is telling you not to do on arch. You need to uninstall all of the packagekit dependices. Then you will be able to use discover safely and only install flatpaks.
Why is it that if they have this information and build these supposedly accurate profiles about you that they would still be willing to show ads at all to the kind of people who are likely to frequent this community? For example, if someone who runs linux, adblockers, firefox with strict profile, etc, etc is being broadcast to these advertisers. Why would they want to bid for advertising space for that person at all?