It is true that python3-dolfin is considered legacy (deprecated), nevertheless the old source is still maintained for the purpose of supporting users with existing projects that have been using the old version of the library.
For more context, note that both src:dolfin and src:fenics-dolfinx are
still current (and distinct) as source packages, in the sense that they
generate a different set of binary packages. src:dolfin generates
python3-dolfin, while src:fenics-dolfinx generates python3-dolfinx (the
next generation version of the library).
