JetBrains provides technical support and regular bug-fix updates for the current version of the IDEs.
Security updates are provided for current and previous versions of the IDEs. The exact list of prior versions that will get an update depends on the severity of the security vulnerability.
Support for the latest OS version is guaranteed in the current version of the IDEs (either immediately upon release or with the bug-fix update). If a new OS version appears after an IDE release cycle, the latest major version of the IDEs will support the new OS version but previous versions of the IDEs may not.
You will find the list of the current IDE versions here: https://www.jetbrains.com/products/.