I am genuinely wondering why the stereotypical Ottoman is depicted with a turban when now a days most Muslims don't wear them and primarily when you see a man wearing one they are Sikh. Was it a religious symbol at one point? Did it die out along with the Ottoman Empire? Are there still other groups of Muslims that wear the turban?
This happened in two phases:
first the turban (which was used as a status symbol) was replaced, in the Ottoman Empire, by the fez/tarbush as a military/civil headdress in the 19th century (there is a law from 1829 on this) - along with the first phase of modernisation of the Empire (abolition of the Janissaries/Tanzimat reforms);
one century later, the fez was in turn banned by Atatürk (1925 law) during a subsequent phase of modernization of Turkey (secularization, Latin alphabet, etc.).
There are still some turban-like coverings commonly worn by Muslims, such as the chèche in North Africa or the keffieh in the Levant.