The only wide angle zoom I know which uses behind-the-lens filters is the Sigma 15-30. The ultrawides like those below 16mm probably uses the similar gel filters as well. No easy polarizer solution for these.
All recent Nikkors below 18mm in primes use 39mm bayonet filters on the rear of the lens.
The Tamron 14/2.8 also uses Gelatine filters (Gels).
Given the ultra wide angles generally covered with sub 17mm lenses. the use of a polarizer really isn't required as in many cases even quite mild polarizing levels lead to noticable dark patches in the sky, the intensity of which depend on the relative angle of the lens to the sun.
Lee Filters (UK) was and probably still is producing a linear polarizing sheet that would fit in a gel holder and similar material was available for years from Edumund Scientific in the USA. However the main problem is that the holder on the Sigma lenses is roughly squarish thus making it nearly impossible to use a gel polarizer for obvious reasons.