It seems like there was a change in the syntax for profiles between Minecraft versions 1.12 and 1.14. Previous versions use .ReAuth.cfg, while newer versions use reauth.toml. As of writing this, both branches are on reauth-4.0.7.

Is this difference anchored to the Minecraft version, or will all versions eventually migrate to using toml? What was the reason for the change?

I'm asking because I want to automatize the creation of my PrismLauncher instances.

Switch to TOML was due to Forge changing the way configurations are written.
Older Minecraft/Forge versions will continue to use the old cfg format instead of TOML for the forseeable future. They may however eventually be switched.
Fabric and eventually Neo will use TOML if possible

Thanks for the answer. Do you reckon this should be added to a README?