Lightning is now the part of the Thunderbird code but it's not always turned on by default. In case you're missing Lightning add-on, instead of trying to install it from the site (where they have the old version for Thunderbird below 60), just do this steps:
1. Go to Preferences->Advanced Tab->General Sub-Tab->Configuration Editor button
Just like with about:config for Firefox, we need to confirm that we accept the risk and really want to open this tool.
2. Search for:
Right-click it and choose: Reset
3. Reboot Thunderbird.
After reboot Event Calendar (and Lightning extension) should be added to the Thunderbird.
The only drawback is that it defaults to English, despite different Thunderbird language. In old Thunderbird, the solution for bringing back localization was to uninstall and install Lightning again. Since we cannot do that and only restore it via the method shown above. This is caused by the fact that localization packages don't have Lightning localization.
Currently, we can bypass this issue with lightning-lang-patcher (from AUR).