Thunderbird 91 was recently released, and this version adds support for Google Calendar without requiring an addon. I use my Google Calendar a lot so was happy when Thunderbird in Slackware current got updated. Unfortunately there seems to be a problem with installing addons on the Slackware packaged version at the moment which results in all existing add-ons being disabled and trying to install a new one gives the error "Thunderbird has prevented this site from installing an unverified add-on" even though it's coming from thunderbird.net.
Until it is fixed I have found the
latest-thunderbird.sh script which you can get here. This re-packages the official Mozilla release of Thunderbird into a Slackware package.
I will probably keep on using that script as I have been with the
latest-firefox.sh script which you can get here since it means I can update Firefox and Thunderbird without waiting for the official Slackware release which can sometimes be a bit slow.
No changes need to be made to either script when a new version of Thunderbird or Firefox is released. The script automatically checks the latest version and creates a package for it. If you already have the latest version installed then it will tell you so and not download anything.