We don't do LTS, not nearly enough manpower. A release is only supported until the next one is stable. However given TC's nature, it's likely any system set up nicely will continue to run for the foreseeable future. I have many such, Rich at one point had a 4.x box running for years, and so on.
Extensions are accepted for older versions, though older than three versions or so may raise eyebrows. Comments about core scripts etc. may still apply to newer versions given the low rate of change, as may questions.
New versions allow access to current applications and hardware support, bugfixes, and since it's now in fashion, cpu leak mitigations.
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Robert famously said that software doesn't have an expiration date. It's totally fine to keep using your current apps until something external forces an upgrade, for example I'm still on several versions old LibreOffice, it does everything I need it to.