What is the appropriate version to study Tensorflow now?

I like the version that matches the learning materials.
If you're a beginner, avoid learning materials that don't have a specific version so you don't stumble during setup.

The library of machine learning has not yet withered and is actively developing.
Even if the major versions are the same, the function names and arguments may change, so it is recommended that you use the same version as far as possible until the minor version.
If you use a different version from the textbook, you may fall for the code you copied because of compilation errors or unintended results.

Also, it may be the right posture to use the same OS, but it may be better to change the teaching materials to a new one than to use an OS that is out of support.