The -dev suffix means that this is developed right now, so it isn’t released yet.
By default you follow the tests-passed branch of Discourse, so like their hosted customers you get all the latest features and fixes when you update.
When a milestone is reached or there is a security fix a new beta is released. Then you are notified that an update is available.
So when 3.4.0.beta2-dev was finished, the release notes which show what was added since the beta1 milestone were published 3.4.0.beta2: Better pageview metrics, multiple category moderator groups, watch chat threads, and more
Everything that was added afterwards isn’t “released” yet.
From Discourse 3.2: adding -dev suffix to beta versions under active development
Understanding Discourse release channels