This week I upgraded my iMac to SSD , completed a fresh install of Mojave 10.14.4, followed by migrating all data and apps.
I added the urbackupclient to the FDA list and turned off csrutil.
I can confirm that, although snapshots are not working, I am able to backup successfully.
UPDATE: I have just successfully backed up another Mojave system, that hadn’t backed up since November, by turning off CSR and adding the client to the FDA list.
MAY 16TH UPDATE: I woke up today to OS X 10.14.5 which is still happily backing up.