Via MacTracker:
MacOS 11 ("Big Sur")
Released November 2020
Most recent update: July, 2022
Mac OS 10.15 ("Catalina")
Released October 2019.
Most recent update: September, 2020
Mac OS 10.14 ("Mojave")
Released September 2018.
Most recent update: July 22, 2019
Mac OS 10.13 ("High Sierra")
Released September 2017
Most recent update: July 9, 2018
Mac OS 10.12 ("Sierra")
Released September 2016
Most recent update: July 19, 2017
However, most people use the internet, such as Safari. Safari 15, released with Monterey, 2021, can work with Catalina. Safari 14, released with Big Sur, can be installed on Mojave systems. Safari 13, released with Catalina in 2019, can be installed on High Sierra.
Conclusion:
1. Once the new MacOS Operating system is released, the one old does not get any updates - EXCEPT for Big Sur
2. Safari is supported for two operating systems prior.