The X-Plane SDK download contains the C headers and pascal interface files to compile plugins, and the stub libraries to link plugins for platforms that need them. You do not need to download any files to run plugins; X-Plane ships with the appropriate infrastructure to host plugins.
Regardless of which X-Plane version you target, you can and should download the latest SDK file above. To support other versions of X-Plane, you only need to change which XPLM macros your project defines, as follows:
XPLM301—X-Plane 11.20 & newer (64-bit only)
XPLM300—X-Plane 11.10 & newer (64-bit only)
XPLM210—X-Plane 10.00 & newer (10.20 required for 64-bit plugins)
XPLM200—X-Plane 9.00 & newer
XPLMxxxdefinition—X-Plane 6.70 & newer
Note that these definitions do not automatically imply older version—so, to target X-Plane 11.20 with all the latest features, you would need to define all of
Here are older SDK releases provided for legacy purposes only. You should not need them. Use the newest SDK above to build all plugins, even when targeting old versions of the sim.