I would say it was an experiment to go along with the Nokia tablet, and you are right it was under Nokia's tenure rather than HMD Global as it is now. As it has been since April 2016 since the app was last updated in the Play Store, then I'd say the product is dead.
Z launcher should be used by HMD as a separated app in the play store under their account, Z launcher is great, I loved for the first time when purchased my nokia N1 tablet and also asked their development team before HMD started operations but never answered to confirm if the Z launcher would once day updated or kept under development
Yes yes yes! I use Z Launcher on my Nexus phones - bare bones android is great because it is customisable, not because it is bare bones! First Nokia, please renew the security certificate for zlauncher.com, then bring the app out of beta and give it a new lease of life.
The website appears to have been recently updated (has a new certificate, no longer expired) and it's now possible to download Z-launcher direct, without using Google Play - nice move Nokia!
It's still in beta on Google Play though, no update since April last year. Oddly Z-launcher has its own feedback forum, it's not part of the new Nokia Beta Labs.