Ok, got it. As ofnuts says it is HSV....
If you take a Gimp 2.8 file and open in Gimp 2.10 the legacy layer modes transfer as this.
Overlay becomes soft-light (l)
Gimp 2.8 example attached:
If you take a Gimp 2.8 file and open in Gimp 2.10 the legacy layer modes transfer as this.
Overlay becomes soft-light (l)
Gimp 2.8 example attached: