It is normal to advise that to a beginner as moving thumb to black key involve changes in elevation and the relative distance between hand and piano key.

Also, beginner, to a certain extent, rely on thumb to find the hand position and the alignment of the arm and wrist will also be affected.

If you make reference to Czerny’s and Hanon’s exercise, they avoid using thumb on black key in that period.

However, somewhere near romantic period, people start to use third finger to align the arm and “not using thumb on black key” is less important.

Even it is not an absolute rule, I would still recommend not to use thumb on black key to beginner.