So according to the knowledge articles that version simply needs to be upgraded to the latest 5.2R5 or rather 5.2.5 version. However, there is no link in sight.
Luckily for us, the University of Exeter has an 'Off campus access with VPN' page that provides a number of VPN clients for you to download.
If you want a direct link to the correct VPN client that will work on Sierra, here it is: https://as.exeter.ac.uk/media/level1/academicserviceswebsite/it/documents/networks/vpn/ps-pulse-mac-5.2r5.0-b869-installer.dmg
After installing, the about box should show you this...
Now you will be able to connect as before.
Another approach is to use an old Junos Pulse 5.0.3 client. I tried that out and it works like a charm, but asks you to upgrade every time you connect to a VPN.