The old C41 stabilizer had formalin, and the old films required it, but the new films don't. So the old stabilzer can be used on both old and new C41 films. The new C41 stabilzer cannot be used on old film due to the lack of formalin.
The E6 process uses formalin in the bleach pre-bath. Therefore, any stabilizer can be used. It is present as the odorless sodium formaldehyde bisulfite adduct. I had the original EK patent on this but I used it in the stabilzer to prevent odor. It was moved to the pre-bath probably to have greater effect. It would also leave no residue on the final product. So, the current embodiment in E6 is better and safer.
I might also add that this shows that the C41 dyes don't need formalin but the E6 dyes do. So those that cross process or use CD4 instead of CD3 and etc, had better beware that the final dye stability might not be what they expect from a given product. Recent C41 products have changed dye forming couplers.