This was once common, but it’s somewhat rare now in my experience, and the upcoming Matrix 2.0 apparently addresses most (all?) of the remaining causes.

I still see it - usual case is when someone has two clients. One of them will have issues with this.

I consider this a good thing, for the sake of the people who joined or wrote in the chat with the understanding that what they write is and will remain encrypted. If you want to abandon encryption, you can always create a new room.

Disabling encryption in the room did not have to mean decrypt past history. Yes you can create a new room. But for big groups who wants to risk it. The room admins I know steer clear of encrypted group chats because of the previous issue.

No, there is one officially released client for android: Element. Element X is in beta. When it leaves beta, it will take over as the one officially released client.

One would never guess based on the release announcement

This is just plain false.

https://spec.matrix.org/latest/client-server-api/#sending-encrypted-attachments

The docs say it clearly “If encryption is enabled”. Otherwise attachments are just a link, nothing special there.