Fennec (<=68) is being phased out in favour of Fenix(>=69).
From that standpoint Fennec will be "frozen" at version 68 and will not be updated to the engine used by the desktop version of Firefox 69.
Fenix will, at some point in the future, replace the current version of Firefox Android and is slated to begin at version 69, with some overlap with version 68.
Fenix features a largely rewritten architecture and engine and brings a lot of potential benefits, but the existing Fennec is pulling too much developer time away from it and causing problems in both camps. As a result it seems they want to draw a line in the sand, call an effective halt to Fennec and move over the majority or developers time.
Android Police have some more information detailing the change.
One of the things not currently supported in Fenix is extension support and it is for that reason that the article you list states that Fennec is "the last Android release with extensions/add-on support."
It is currently unclear, as far as I can see, as to what the intention actually is with regards to extension support in the future. It may well be that if we choose to upgrade in the meantime that extensions are lost temporarily but, short of Firefox following Googles lead with regard to crippling the extension system, I would expect them to return at some point.
It does appear that there is still some discussion going on and that feature parity with the current version (Fennec/68) is intended.
The mobilisation of developer time might actuall turn this feature from "nice to have but low priority" to being actively worked on.