The latest upload still doesn't work on Catalina. Seems like the executable doesn't have the executable bit set.
I got it to work by doing this (bit involved):
Open up terminal
1) Got to the folder where you downloaded the game and enter the Contents/MacOS folder, for example:
cd "/Applications/itch.io/Art Sqool mac.app/Contents/MacOS"
Your folder might be different than "/Applications/itch.io/"
2) Set the executable bit then try to open it by running these commands:
chmod +x "Art School 2k19"
open "Art School 2k19"
MacOS will complain that it can't be opened. Click OK. Don't close the terminal window yet.
4) Go back to Finder and find the game. Right click or Ctrl-click on the 'Art Sqool mac'icon. Select Open in the popup menu.
MacOS will complain again.
5) Go back to the terminal window and try to open again.
open "Art School 2k19"
6) Go back to Finder and open like in step 4.
This time select Open when the popup window appears.
7) Attend Art Sqool! :)