This brand claims to be fully recyclable:

https://www.koehlerpaper.com/en/products/Thermal-paper/TH_Bl...

It supposedly works by using a layer of reflective bubbles that collapses when exposed to heat:

https://www.youtube.com/watch?v=pc1mlCThevg&t=20s

Of course, I don't know how those bubbles are made or how they are made to collapse from heat, so for all I know that layer still uses chemicals that currently slip through regulations. But I can also imagine that it is doable to create such a "functional layer" with safe materials.