No there isn't
To be perfectly honest even though it is embarrassing, I have used 1.5.x for so long that I thought 1.5 was stable!
In fact, I went to the website to read it since I was so shocked by what you wrote.
I do like the message/header caching ability, smtp built in, and easier list/subscribe commands.
Since 1.4 is stable, no reason to change. I'll just install msmtp or something else, and edit my .muttrc
Thank-you very much.