NFL only mandates then during practice, IF you have a lower tier yellow or red graded helmet. Optional during games matter what helmet.
Sounds like they are waiting for everyone to lock into newer helmet technology or GREEN tier helmets which
According to NFLPA
"High-Performing Helmet Exemptions: Players do not have to wear a Guardian Cap during practice if they choose to wear one of the specific, high-rated newer helmet models tested to provide equal or better standalone protection."
So you can basically choose to wear a helmet that test just as good or better than the guardian cap