The most mainstream phenol free thermal paper out there is one made from a branded chemical "pergafast" which is supposedly based off of urea. This is increasingly the type used by big brand stores where the paper is advertised as BPA and BPS free. It appears safe in studies so far in terms of not being absorbed through the skin nor endocrine disrupting, but is also known to be highly toxic to aquatic life. There's another chemical related to vitamin c branded "Alpha Free" but it's harder to find.

https://www.sciencedirect.com/science/article/abs/pii/S02732...