The file is protected by System Integrity Protection as outlined here.
By disabling SIP you will be able to edit the info.plist again.
So boot to Recovery Mode by pressing cmdR while booting.
- Open Terminal.app (menubar -> Utilites -> Terminal)
- Enter
csrutil disable
- Reboot your Mac
- Edit the file /System/Library/CoreServices/Finder.app/Contents/Info.plist
- Reboot again to Recovery Mode and enable SIP again:
csrutil enable
- Reboot your Mac
You should also be able to edit the file directly after booting to Recovery Mode without disabling SIP:
/Volumes/name_of_main_volume/usr/bin/nano /Volumes/name_of_main_volume/System/Library/CoreServices/Finder.app/Contents/Info.plist
You may have codesign Finder.app afterwards:
sudo codesign -fs - /System/Library/CoreServices/Finder.app/