I agree 50mm is not a "portrait lens", but it does depend on the definition of "portrait". Head and shoulders? Full length? A group shot? It depends on DX or FX too.
The camera standing too close to the subject gives distorted perspective, which tends to enlarge near noses, etc. Ladies notice this result, and they don't like it.
No one ever wants to hear it, but the ONLY CORRECT rule for proper perspective of portraits is to use whatever lens that lets you stand back 6 feet (or more) from the subject, and then get the view that you want. That is easy, and the right attitude is to be damn glad you know this.
For 35 mm film (FX), 105mm was always considered the ideal "head and shoulders" lens.
Because, head and shoulders requires nearly a two feet height. The angle of view of 105mm is about 19.5 degrees (long dimension).
This combination (105mm, FX, 2 foot view height for head and shoulders)
forced the camera to stand back about six feet, which was a good result for perspective (does not enlarge near noses, etc). Could do no wrong, so to speak. Many photographers favored more distance, like 135mm and eight feet (speaking FX) for a more natural undistorted view. But less than six feet is a no-no.
For DX, 70 mm is the equivalent view of 105 mm, so 70mm DX would also be appropriate for head and shoulders (forces standing back at least six feet).
For full length, a wider lens is necessary. For group shots, even wider.
But the ONE ABSOLUTE RULE for any situation is to first always stand back at least six feet. Then you are safe there (perspective-wise), use whatever lens that gives the view you want. Which lens is NOT important. What is important is standing back at least six feet, and then getting the view you want.