"Mods" Folder worked once, then no more?

There are no changes required for Win 10. If you are looking for a fresh copy of the Mods Framework, that would be here:
https://modthesims.info/wiki.php?title=Game_Help:Installing_Sims_3_Package_Files/Setup_and_Files

Sometimes what happens is that your system ends up with a second unwanted TS3 user game folder, so that the one into which you are adding mods and other content (even though it's in the correct place) is not the one that is being read from when the game loads. A common place for these additional game folders to hide is in OneDrive, as there would be a Documents folder in there that the game might confuse with the real one. On Win 10, OneDrive tends to be activated by default or at least the user is nagged during setup to activate it.

Thank You, igazor!