FlexSim models saved in a newer version are not compatible with earlier versions. For instance, a model saved in FlexSim 26.0.x will not be able to be opened in FlexSim 25.0.y. This does not apply to bugfixes of the same feature release (for instance, a model created in 26.0.3 can be opened by 26.0.2), but in general you should always upgrade to the latest bugfix release of your licensed version. For this reason, the best answer is for you to upgrade to the newer version.
To directly answer your question, we only have one LTS version at a time. Currently that is 26.0.
Software Support Lifecycle - Autodesk Community
