Curious about the Mail app on iPhones or the Mac. Does it allow filtering of incoming messages based on user-defined rules? You know a la Thunderbird?
That ability is incredibly useful. I don't find that feature on Mail on the iPad and didn't find it several years back when I tested Mail on the Mac.
Has it ever been added?
Yes. You can add as many custom “rules” to incoming email as you like, using macOS. I have a rule setup to shunt any incoming iTunes purchase invoices to a dedicated mail folder, for example. I don’t think iOS has an editor for those rules, but they are honoured on iOS as well, once set up.