We now ship this new upgrader script in 9.0.54 and 10.0.1. If you want to test this in previous versions use the steps from Help testing the 9.0.54RC1 and 10.0.1RC1
To see if you are in the correct version of the updater check out the two pictures in https://github.com/nextcloud/updater/issues/39#issuecomment-250399443
I will close this ticket here now. Thanks for all your feedback.
@nicojack: try the steps I linked in the first paragraph.