Found this post from Thoth after missing it in 2019. ;-)

With steemit and steempeak my attempts to send an encrypted message were in vain, the message was not encrypted.

This was surprising to me. I am pretty sure that I've tested encrypted messages from Steemit sometime in the past, but I tried today, and it seems that encrypted memos are disallowed by the Steemit wallet. It doesn't send unencrypted, but it silently refuses to broadcast the transaction, and just returns me to the previous screen.

6 years late, but thanks for expanding my knowledge. ;-)

Edited to add:

This comment cleared it up for me. It's something with Steem keychain. If I use the Memo key directly, without Steem Keychain, it works.