Also, if you are not aware, Pd vanilla ships with the "Deken" externals installer plugin: Help - > Find externals
The centralized externals included with Pd-extended have been split up and some are being maintained and new builds available via deken. If you know the libraries in your old Pd-extended projects, it's likely that you can download them.
One thing to know, is that the old [import] which automatically scanned and loaded all libraries is not part of vanilla. The preferred approach is to use the new(er) [declare] object to add paths or library names, sort of like including header files in programming languages. This way, there is no ambiguity when sharing patches with other people.
We as a community are slowly working through this transition process and there are a few details to work on, but it's largely a good thing as it helps split up the work and external development activity has increased in some areas.