The foot is defined as exactly 0.3048 meters, does that mean it’s an SI unit?
If you look at section 2.3.4 in the link below you will notice that there are no scalars in those derived units.
They will all have the form:
x^n * y^m * z^o
Also note how they define a 'degree Celsius' which is equivalent to a degree K, but then invoke a T_0 to include it to avoid breaking the above rule.
https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-...