It's really not a good idea. You will almost certainly lose data, likely in a way that prevents import into the extremely old version. Why do you need the data for such an old, obsolete Spine 2.1 version?
Do you have the 2.1 Spine project files? I assume you don't, but I have to ask.
Using Spine 4.1 you can rollback to 3.6 on the JSON export dialog. Going back any further is not supported. You may be able to use the 3.6 JSON with an older Skeleton Viewer JsonRollback. If the JSON you get cannot be loaded by 2.1.27, you may want to try deleting timelines that aren't supported in 2.1. First try deleting all timelines that aren't rotation, scale, translate, or shear. If that works you can narrow down which ones are giving you problems.
Lastly, you can setup Skeleton Viewer development and debug way the JSON cannot be loaded. You may then be able to figure out how to modify the JSON to load. The animation is unlikely to look correct, especially if scale was used, but may be salvageable with a little work in Spine.
