Old compose mode is no longer available
Google has started dropping the old compose mode. Thus, you’re no longer able to switch back to the old compose mode. Luckily Mailplane 2.5.12 and Mailplane 3 are already supporting the new compose mode.
The new compose mode has been launched at the end of last year and Google improved it on a continuous basis since then. Check out their sum up to learn everything that’s important about that new compose mode.