I think it is a myth lemme tell you why:

Shoulder length hair is hardest to care for. The ends are brushing against your shoulders all the time which causes friction. You really have to protect your ends.

How do I know?
I was told at thirteen (by ignorant ppl) that my hair would never grow passed shoulder length.
I ended up doing protective styles. Just buns because I didn't know how to braid at the time. And I stopped using heat. COMPLETELY.

Now at 21 after I BC'd in 2009, I have midback length hair. Hip length when straight.

Just take really good care of it while it's at shoulder length.
Mostly 3b with 3a and 3c underneath
Layered but mostly BSL (Hip length when straight)
Conditioner: As I am Cowash, OR Tresseme Naturals OR Shea Moisture Masque.

Style: KC Knot Today+ a TINY bit Shea Moisture Curl Smoothie.
2nd-6th day hair: Spritz with a mixture of Aloe Vera Gel, Coconut Oil, and water.