I believe “Navajo” was in common use long before anyone applied for a copyright. The law-maggots might not care and the Pharisees they work with will rule “no” but common use is fair use in any reasonable mind.
First of all it is a TRADEMARK and not a copyright. Trademarks do not expire. Copyrights do. The Navajo Nation has a legitimate recognized trademark for their hand crafted items that they want to protect and this law only applies to arts and crafts and not to generic items.
For instance, if you wanted to use the term Navajo for a brand of motorcycle, then the trademark restriction would not apply.
I side with the Navajo's on this one.