The logic for determining item quality changed a while ago (back in version 0.40, I think), and I believe the significance of Legendary+5 has been largely lost in the process. A few mechanisms still use it (such as Farming/Herbalism yields), but item quality seems to use a completely different formula now. Further research is needed. --Quietust (talk) 13:28, 6 May 2020 (UTC)
This section suggests there is no effect other than the rusty skill description in fortress mode. By observation a rusty skill might effect quality of produced items and eventually the dwarf will loose experiece and/or skill level.