There exists a 'Relighting Skyrim' and a 'Relighting Skyrim - Updated for USLEEP' mod. I am using USLEEP (unofficial legendary edition patch). In the description of the USLEEP version (mod order) it clearly shows both versions (the original relighting skyrim and the USLEEP version) being used. Like if the original was needed and the USLEEP version just modified it. However, both versions have the exact same name.

Was the 'Updated for USLEEP' version changed so that the original 'Relighting Skyrim' is no longer needed?

TIA!

Load em up in TES5Edit and report back with your results. It should be fairly obvious.

Yes, the mod was changed so that it no longer needs the original. It will cause problems if you try to have both loaded (as I found out accidentally). The plugin is now called simply RelightingSkyrim.esp. It is still a WIP and has been getting updated pretty often. The author is working under the assumption that the original is abandoned (again).