Definition 2.
In a more substantive tone, I personally see little value in 5 alpha versions. If none of them progressed to beta or an RC stage, they don't really provide much utility. Why would I try to ship a product unsuitable for production? I fail to see value in a product promised, but as of press-time, yet to be delivered.
Regardless, we can simply look at other frameworks to compare. Foundation 6 is out and is pretty solid. There are various material-design inspired frameworks, and more barebone grid systems that do everything bootstrap 4 can do with it's grid system. Did I mention many are lighter-weight and also production ready?