The only ads I ever see in either Google or Proton web mail clients are for their own services, or if I check the spam folder. And I don't have an ad blocker set up (I have NoScript, but there isn't any third-party JavaScript anyway).

I use webmail clients nearly every day and I don't see ads.

Do you Yahoo!?

No, I use Google Workspace and Zoho and Roundcube for my webmail.