Many plugins and tools have been available for a long time and support many older releases. Use the Marketplace entries to determine availability of a version supporting the Atlassian product release you need. As a general rule, testing and support is done on at least the last 3 Atlassian product releases. Atlassian Command Line Interface (CLI) has a more complicated compatibility matrix - see Compatibility Notes for more details.
Any operating system supported by Atlassian products. Windows, Linux, and Mac OSX are supported clients for Atlassian Command Line Interface (CLI) (plus related CLIs) and GINT - Groovy Integration Test Framework support .
We support Atlassian platforms running the Java version required by the platform except where specifically noted. Our primary test platform is the minimum Java version supported by the Atlassian product version. Oracle has ended public updates of Java 1.7, so we strongly recommend using Java 1.8 for all client and server environments (except on any older release where it is not supported).
Viewing your Java level
Most Atlassian products have a System Information panel for Administrators that has Java and other information.