Thanks Kulkul,
I reported the issue on GitHub - the developers said that it is ‘agreed practise’ to only support 3 versions. So dropping 2016 support was automatically done without any discussion. I think I upset them by complaining about this issue, but I see that other people are now complaining about this too.
While Autodesk decide whether to do anything about this issue, I have found my own dodgy workaround. I found a post on GitHub where some service pack for Revit 2017 did the same thing - someone suggested copying the Dynamo install files for 2016 to a safe place before installing the new version of Revit/Dynamo - then copying them back afterwards. In my case it was too late, so I found the installed dynamo files on another computer and copied them to the computer where they’d been blitzed. Much to my surprise it worked, and I now have Dynamo 1.2 on Revit 2016 and Dynamo 1.3.3 on Revit 2019. The folders are:
C:Program Files\Dynamo\Dynamo Core\1.2
C:Program Files\Dynamo\Dynamo Revit\1.2\Revit_2016
Then copy the ‘Dynamo.addin’ file from:
C:\ProgramData\Autodesk\Revit\Addins\2016
Copy those folders and files back to the equivalent place after the Revit 2019 installer has done its stuff.
I can’t promise it will work for you; and please be very careful messing around in the program Files folder (I wouldn’t normally recommend it).