In 2.83 and previous versions there was a bug that was causing boundary
vertices to be detected incorrectly that was preventing the smooth brush
to work on boundaries if there was a pole on them.
In 2.90 the boundary vertex detection was fixed, but it was still using a
simplified version of the algorithm without any boundary smoothing. This
patch implements a similar smoothing algorithm to what I think it was
the intention of 2.83 and previous versions, but working correctly.
2.83 smooth, this is why the default smooth algorithm was changed:
2.91 after the fix: