I was getting the error "Error downloading https://raw.githubusercontent.com/damel ... index.json"
...and all the ATTiny options were missing from the boards menu.
This (eventually) is a fix I found:
Open the IDE and click File, Preferences.
Click the link on the line after File > Preferences > More preferences can be edited directly in the file:
This should open a folder named something like Arduino15 (the exact name depends on your OS).
in my case it was "C:\Users\Phil\AppData\Local\Arduino15"
Close your Arduino IDE, then go back to the preferences folder.
Move all files in that folder except preferences.txt to a temporary location (as a backup).
Re-open the Arduino IDE.
Opening Tools, Boards, Boards Manager again will force a reinstall of all the board specs - the missing boards should reappear and the download error should be cleared. Worked for me
Cheers
Phil