Gee I'm still on version 11. I tried updating once, got a ton of useless crap like calendars and shit and reverted back. Email is one of the few things that I want something very basic and crude.
In addition to the point others made about vulnerabilities, you should know that Thunderbird currently split the calendaring features into an add-on called Lightning that is included but disabled by default. If you don't want a calendar, you won't get a calendar.
Careful. Old versions of internet-facing applications like email clients are dangerous and put you at risk. They often have widely known security issues which can allow malware to access your PC.
I'd advise upgrading Thunderbird and ignoring the features you don't use, or switching to an actively-maintained mail client which is designed to be simpler than Thunderbird.
Seeing comments here of people who want mobile-like or web-like features in Thunderbird, I feel old school, but daaamn, 11? That's 6+ years of piled remote vulnerabilities. Clearly Thunderbird is not to your liking and you should try other clients.
