I think a open-source exemption would be acceptable (if done properly; another comment mentions a possible problem), even though I would think it would be preferable to not have such age-verification bills at all. At least, adding the exemption would be second best, which would be better than having the age-verification bills without a open-source exemption.

(As another comment says, it is still not good, but at least it is something.)

This is the "lesser evil" trick politicians use to silence the opposition.