PBNs era will end when Google will stop caring about backlinks. Which is not soon. Also, properly set up and nurtured PBNs are way, way, WAY better than guest posts and niche edits, even for the only reason that you have control over them. With guest posts and niche edits the owner of the site that gives you the guest post / niche edit link can simply decide to remove / replace your link, or their site may get penalized and then your link will lose its value, or the owner might go out of business and you will lose your backlink, or they might decide to nofollow your link and your link will lose its SEO value, and so on. Many things can happen with guest posts and niche edits. PBNs - again, when done right - are way more valuable than all other types of backlinks, so I'm not sure what you're talking about.
OP: PBN means Private Blog Network and it's basically a bunch of websites that you own. Any site that you own is technically a part of PBN because 1) you own it (so, it's private) and 2) it's a blog. So, any site that you own and that you can place links on that point to your more important sites (which we call money sites, and which are sites you wish to rank in google because... well, they make you money) is a PBN.
PBNs also include money sites because the evolution of PBNs has gotten to such a point lately that you need to treat them just like money sites in order to get the most value out of them. A website that ranks and gets organic traffic will be more valuable as a PBN than a site that's not even indexed in google, even though, technically, both of these sites can be called PBN. This is, in a nutshell, the deal with PBNs.
And yes, if Google find out that you're using PBNs to improve your rankings be sure that they'll penalize you (deindex all of your sites in that PBN, including your money sites). But, since many people rank and bank with the help of PBNs it means that Google can not detect them IF they are set up properly. Here is a guide on how to safely set up PBNs:
https://www.blackhatworld.com/seo/the-ultimate-pbn-guide.1360359/