According to the WWDC Keynote this is now possible; however I cannot figure out how to do it.
How to change the default email client on iOS 14?
I just spoke with someone from the App launching and linking lab. At a high level, this is what I learned.
the documentation, etc isn't done yet, but they are working on it
there isn't a specific date when they will have the documentation completed. But certainly before iOS 14 release.
there is no session
like some other sensitive features, you have to get a special entitlement in order to be able to do this
send an email to default-app-requests@apple.com to have them review your app and grant you the entitlement
they will review your app to make sure that you are actually an email client, can send/receive emails and not a game, etc
your app can't have both the email client and web browser entitlement, you can do one or the other
the setting to switch which app is the default mailto handler doesn't show up until there is more than one choice on the system
the setting appears under the settings for the 3rd party app
there is a runtime check for the presence of the entitlement and that mailto: is handled
the mail client entitlement is set in the plist as com.apple.developer.mail-client=true
the web browser entitlement is set in the plist as com.apple.developer.web-browser=true
the web browser has to be able to handle both http and https requests, and a few other things
this should be possible in the current beta 1 code