No, I believe it's just natural variation, this was far off in the woods, and there are no other Yucca palnts in the area with elaves equal or as narrow as this one.

Also you can see Flora of NA shows Yucca flaccida can have a leaf width between 1 cm to 4 cm and (sometimes as wide as 5cm). This Yucca was down to around 1 cm, the narrowest range they list for Y. flaccida:

http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242102063