With this it’s up to client whether to display symbol near display name like former checkmark, or as a badge or what.
I meant, this envelope emoji would be displayed in the colored avatar placeholder like it’s done currently for contacts having encryption keys (for them the first character of their name is displayed). This, in particular, doesn’t require additional work on the UI side, but still, switching to a new API is some work and the new design should be agreed with the UI teams, so at the moment it’s only a proposal (one of many) and can’t be implemented quickly.