I noticed the beta launch of Engineering today, and I was confused; I received emails previously for such events, including the beta launch of Moderators (now Community Building) and the commitment phase of Engineering. I checked my spam, junk, trash, and could find no emails from Stack Exchange around the time of the private beta launch (nor public).
At some point I noticed a link in my Area51 profile prompting me to verify my email address. You see, some time last year I added a second login to my SE account so that I could authenticate using my Google credentials. However, I must have already verified the GMail address, as evidenced by the fact that it received an email as recently as January 7 notifying me about the grace period for awarding a bounty I had offered on Stack Overflow.
Somehow I was able to verify my GMail address on the network in a way that did not carry over to Area 51, and as a consequence, I stopped receiving emails about proposals. It may be significant that the bounty email came from stackoverflow.com, while the emails I've received relating to Area 51 have all come from stackexchange.com.
Once I noticed, I was able to verify the address on Area 51. The following week I received an email to that address announcing the public beta. Had I not noticed the absence of an expected announcement email, I might never have realized that the email address I added once (according to memory, at least!) needed to be verified twice.
I suspect part of what happened is that, after adding the new login credentials, I changed the email address in my profile from another site in the network and applied the change from that one site to "all" communities. I've checked the other network sites on which I'm registered, and only Area 51 appears to have required me to verify the address again.