macOS Monterey (version 12), that was released last year (in 2021) is the last one. Right now, the latest major update for this branch is 12.6.1 (21G217) and it was released on October 24, 2022.
This OS should get security updates for at least two additional years, so at least until 2024.
Every modern Apple OS is normally supported around 3 years after the initial gold release.
However, here is an interesting article:
Some Macs are getting fewer updates than they used to. Here’s why it’s a problem
Dropping old Macs can be justified, but some are dying before they should be.
arstechnica.com
Here is a quote from that site:
- Macs that are sold for an abnormally long time—the 2014 Mac mini that was available until 2018, the 2013 Mac Pro that was available until 2019, or the 2015 MacBook Air that was available until 2019, to pick three examples that Ventura doesn't support—don't get software updates for longer just because Apple sold them for longer. This differs from the timeline Apple uses to provide hardware repair services, which is determined based on "when Apple last distributed the product for sale."