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.