Technically, this 15.1 release is also alpha. There is usually one or two alphas and one or two betas before final release.You can when this update it final. This is the ".1" beta version of this update.
They do all their updates the same:
XX.0 - alpha update - rough new quests/items in place, not fully funcitonal
XX.1 - beta update - new quest/items more polished, and alpha bugfixs, no voice acting for new content yet
XX.2 - beta+ update - voice acting added and beta bug fixes. sometimes they do a XX.3 and XX.4 update if they need that many bugfixes.... after that they push the final update for that version to steam/itch.io/etc.
But sometimes there is just one final release for low content updates. Like 0.12.4 that was following 0.12.3.