Different versions of WoW *do* have their own directories for addons. However that’s not different versions as in 11.0.0 and 11.1.0, or even 10.0.0 and 11.0.0 - that’s retail, two different classics, and PTRs, which only share game data and not configurations or addons.
By my count the base game has 1,721 directories (1,631 of which are in Data); though I haven’t done a clean install in quite some time. Extras then exist on top of that for each addon, plus accounts, realms, and characters that have been played locally. Every extra “version” (per the above) is another 23 directories.
It’s also worth mentioning that many addons will, at least by default, install their libraries privately. This will result in a great deal of duplication, though can be necessary at times if a library has been updated but the addon has not. Often you can get by without this, using “public” libraries instead that only get installed once per version.