What does it mean that the Network Engineering site is in a beta-phase and what factors into the removal of its beta status?
SE allows the community to have a large role in deciding which topics there should be sites for in their network. They use the site Area 51 to allow the community to submit ideas and gain backing.
There are a couple of stages on Area 51, and if an idea passes all the criteria of each stage, then it is given a chance at a beta site.
A site stays in beta until the metrics of the site indicate it is a quality and self-sustaining site. You can read a couple of older blog posts about this process here and here, as well as a meta question about it. While there is mention of 90 days in these posts, keep in mind that there are sites that have been in beta for over 1k days.
You can find the Area 51 page for the Network Engineer proposal here. Our site continues to show growth, we just need more time to develop.
If a beta site gets to a point that SE feels it will not ever reach that "critical" mass, the site can be shut down. Content is still available for download and can be migrated into the site (or sites) that is most appropriate. See, this post or this post for examples.