Their top end product, H&R Block Tax Software Premium & Business, does not support macOS. Seems odd the other 3 versions of H&R do support macOS but not the Premium version.
I was able to use hexedit to NOP out the OS version check call and got TurboTax 2018 to start up without displaying the "no longer supported" error window. It still warns that next year's application will require 10.13, but I will worry about that next year. Here is the gist of what I did -- assumes a basic knowledge of terminal and simple hexedit commands:
% cd /Applications/TurboTax Home Business 2018.app/Contents/MacOS
% cp TurboTax\ 2018 TurboTax\ 2018_backup
% hexedit TurboTax\ 2018
If I were to hazard a guess it likely could be they are being cheap and only testing with the latest OS available. Might be time to consider finding a new tax package. < sigh />