Using this flag from linked data is always a double-edge sword, in one
end some user have been relying on it to keep around data that is not
really used as ID (like e.g. text data-blocks, node trees, see
e.g. #103687, #105687). On the other end, it often causes over-keeping
of linked data reference in production files.
From now on, when an unused linked data is to be kept around, users
should create an ID property to reference it.
Implements #106321.
Pull Request: https://projects.blender.org/blender/blender/pulls/111042
f052b18a65