I tried to open a blend file in blender 2.8 that was made in blender 2.92 version. After opening the file,in the shader editor,the mapping node is displayed as undefined and has no effect.
Later versions of Blender introduce additional features and the developers try as much as possible to make these features compatible with later versions moving forwards. This is possible since each later version of Blender can be aware of the capabilities of earlier versions and can therefore “know” how to handle the features.
The opposite is not the case - older versions of Blender are incapable of handling features that did not exist when they were written. There is no project to go back and modify earlier versions to include all newer features (why would there be? Just use the later version!). Therefore, when you open a .blend that was saved from a later version using an ‘old’ version of Blender there may be features in that file that the ‘old’ version is not aware of and cannot handle. The developers do a good job of handling this as best as they can (the old file is generally loaded in a limited fashion, rather than just “crashing out” and refusing to load anything at all) but it can’t be expected to handle features that didn’t even exist at the time that version was developed and released.
Various nodes have changed drastically over the versions and the Mapping node is one of those that has changed quite significantly (it has inputs now that it didn’t have at earlier versions). From Blender 2.8’s point of view it is a completely different and unknown node.