Is it OK to delete older versions of Windows 10 SDK?

If storage space is a pressing issue for you, you can proceed to delete the older SDKs. Deletion can be done via program files in pre Windows 10 Creators Update builds or from Settings in newer Windows 10 builds. However an ideal software should provide a good backward compatibility at least within a major version of Windows which at this point is Windows 10.

Imagine someone with an early build of Windows 10 being unable to download your app from the store. You lose users/customers just because they have to update their OS all the way which many avoid altogether.

Keeping the older SDKs will allow you choose a minimum Windows 10 build that your app should support and hence allow installation from devices on those builds.

You can uninstall the older versions via Add/Remove Programs. It maintains a different entry for each version.

If you need to target specific builds of Windows, then you might want to keep older SDKs around. As new SDKs come out there may be functionality added that isn't supported by older builds of Windows, then your code will not run on that build. Generally you want to use the newest SDK available that works for your targeted audience.