Apple seems to support macOS release for 3 years. 1 year while it is the current release, and then for 2 years with security updates after it's successor is released.
macOS Sierra released in 2016, moved to security updates status in 2017 (after the release of High Sierra), unsupported in late 2019.
macOS High Sierra released in 2017, moved to security updates status in 2018 (after the release of Mojave), unsupported as of November 2020.
macOS Mojave released in 2018, moved to security updates status in 2019 (after the release of Catalina), probably will be supported until late 2021.
These are just a few examples. This applied to El Capitan, Yosemite, Mavericks, Mountain Lion, etc. I'd expect Catalina to be supported for the next 2 years (late 2022) and for Big Slur to be supported until late 2023.