bionic's libpgplot-perl used libpgplot0, a partially-compatible replacement for PGPLOT based on Giza, instead of pgplot5. Nothing has changed since then. pdl should not bundle any pgplot or binaries, since that is not how packaging systems work. If one wants to use PGPLOT with pdl, then one should install libpgplot-perl.
bionic's libpgplot-perl used libpgplot0, a partially- compatible replacement for PGPLOT based on Giza, instead of pgplot5. Nothing has changed since then. pdl should not bundle any pgplot or binaries, since that is not how packaging systems work. If one wants to use PGPLOT with pdl, then one should install libpgplot-perl.