"Archetypes will remain available through the Plone 5 series" [1]
What does "available" mean exactly? Will my Archetypes-based custom content types still work (add/edit/view) in Plone 5?
Some of my custom content types inherit from ATContentTypes; for example, I have "class SpecimenFolder(folder.ATFolder)". Is this thing no longer viable?
If I create an empty Plone site with that same "SpecimenFolder" add-on, suddenly the "createObject" link for SpecimenFolders is a 404. Am I doing something wrong or should I re-write "class SpecimenFolder" to be "class ISpecimenFolder" Dexterity-style? And if so, how do I migrate existing SpecimenFolders in a site to this new custom type?
Plone 5.2 (currently in alpha) is going to run on both Python 3 and Python 2. Archetypes is not going to be upgraded to work on Python 3, so if you still need to use Archetypes in Plone 5.2, you'll need to run it with Python 2.7.
The migration from Archetypes to Dexterity can be done multiple ways:
If you are building a new site/buildout, you can migrate the content over using Transmogrifier. See https://training.plone.org/5/transmogrifier/ (note the bobtemplate mentioned in the training hasn't been completely released yet, I hope to finish that this week)
Chrissy did a good job summarizing your options for migration.
In addition: If you create a site on Plone 5.2 (still on Python 2) and install ATContentTypes as an add-on before installing your add-on that uses Archetypes, there's a decent chance it will work. The core team isn't actively trying to break Archetypes support and has gone to some lengths to make sure the tests keep passing in Plone 5.2 (for Python 2 only). But, it's no longer the focus of the core team, and it's unlikely that support will be continued beyond Plone 5 or that the core team will port it to Python 3.
The official story for integrators and customers is that no production environment should run on Python 2.7 after 1.1.2020...while this is not going to happen in general, the general advice is to get rid Archetypes-based code and add-ons in Plone 5 projects asap. This also means the death of fancy add-ons like PloneFormGen.