I had some trouble to install the latest ATTinyCore in Arduino 1.8.9, following the directions on the GitHub site.
Adding this URL
http://drazzy.com/package_drazzy.com_index.json
to Preferences > Additional boards managers list accomplishes nothing, despite changing the order of entries, restarting the IDE, etc. No additional boards appear in the boards list, and no error messages are emitted either. I have two other entries in that list, for the Pololu Astar boards and MiniCore, which work fine.
Problem solved by manually placing the unzipped file in the sketchbook/hardware directory as instructed.