Sorry to be a bit harsh, but to focus on the leg color of the Slaty-backed Gull (SBGU) based on from fuzzy poorly lit photos is a great silliness. Comments based on experience with a single bird is rash, at best. For instance, look at the SBGU at https://ebird.org/view/checklist/S32827708
Leg color is famously variable in gulls, based on age, hormones, even temperature. In the Pacific NW, where these beasts are regular, we paid little attention to the leg color of a potential SBGU. And also note that observers of this bird did note that the legs were brighter pink than surrounding Herring Gulls
Indeed, as with any gull, confident diagnosis is based on a set of marks, as any given mark is variable within any given species.
The short wings, presence of a secondary skirt, dull yellow and proportionately gentle bill, large chested and small headed build, prominent dusky smudge around a staring white eye, pink legs, broad tertial and scapular crescents, broad white trailing edge to wing, and primary pattern (some of these marks seen at close range, others at distance, many both) form basis for a solid ID
Part of the problem is that there have been a number of reports clearly NOT referring to the original bird which had minimal red and no black on bill.
I have not seen a convincing description of this bird since 23 December, which does not mean subsequent sightings have been in error. Clearly, birds with big bills, prominent red, or any black refer to a different bird than that originally seen. Birds with bulky heads, likewise.