Non-chatmail is supported. Especially if you have your own domain or small email server managed by you or local admin, you may prefer using it over chatmail relays. The only function that depends on chatmail currently is push notifications over Google FCM and Apple Push Notification Service are not supported in this case. This may change with draft-gougeon-imap-webpush-03 and mox server implementation of it, but don’t rely on it happening soon.
Autocrypt support is not removed. Unencrypted messages still have Autocrypt header, so if you send a message to Thunderbird user, Thunderbird user can import your key and send you an encrypted and signed message back. Receiving such message should result in a creation of a new 1:1 encrypted chat. What is missing is the upgrade path for two Delta Chat clients who have only shared an email address:
Encrypted group chats, even non-protected/verified, are converted to encrypted chats. If the chat is converted to unencrypted during upgrade, it is either because it was not encrypted at the time of upgrade (one of the members did not have an usable key) or because the chat was originally created from a non-chat message. Other cases are likely bugs.
If you want to see better support for interoperability with other clients or unencrypted email support, the best is to create fine-grained topics with a particular use case that you think is not addressed and exact steps for reproducing the issue in case of bugs.
When bugs are reported with enough details, they result in fixes, e.g.: