What's worse is that Whatsapp doesn't tell you if they changed the key, so there is no protection against MITM.

You need to enable security notifications, and you will be notified of key changes.

> Turn on this setting to receive notifications when a contact's security code has changed. Your messages and calls are encrypted regardless of this setting.

WhatsApp used to. Did they stop?

I don't use Whatsapp, so I don't actually have first-hand knowledge. Now that I think about it, it may be that I was told that it automatically regenerates the key more often than they need to, which would have the same effect.

I don't believe this is true. Every time I've asked about a key change, it has been because someone either got a new phone, reinstalled the app, or reset their phone.

It's a setting which is off by default.