Yes, it is.
I have worked with R on Windows Server for many years with no problems
except for the usual ones, users make something wrong, configurations,
tool chains needing upgrading, etc. But nothing unsolvable.
Give it a try and I'm sure that you will be find it that R is great
software with no real Windows Server 2019 compatibility problems.
There is one volunteer-supported installer for all Windows versions, and it is not built as a Windows Service. I highly doubt that any of those volunteer developers have access to Windows Server 2019... if it doesn't work there interactively then you may need to enable some level of compatibility using OS configuration tools.