Comment 5 for bug 1903689

OK, I found some relevant commit in requirements for stein, commit 6b67b6406c939df2bf706b6525ec123546a73b3e

    Add neutron to stable upper-constraints

    Some projects, notably neutron plugins, are using the neutron service as
    a library. There is the neutron-lib package, but apparently they have
    not moved everything needed over to there yet.

    This is being added directly in stable branches only. Master needs to
    get the latest version available, so these will need to be added at each
    release so the stable branches use the correct version of neutron for
    their respective stable release.

So it looks like we did not apply this step in branch creation for train and newer, I will send patches