I mean, we can, because we have converters.
So, we can create content on 3.3.5 (but only custom) maps, m2s, wmos and convert it to elder version of WoW.
Big problem for me it's WoD database (dbc/db2 etc) but I know it is real. Because some guys made moded WoD client for 3 month. With custom PvP non-class system, they remake interface etc. So its real, but we need more tools for it not for 3.3.5.