Do women working in the design and tailoring of women's wedding dresses constitute deeds that lead to Hellfire, and is it forbidden by Sharia?

It is permissible to design clothes, however, it is not permissible to sell them to one who is known or highly likely to wear them immodestly in front of non-mahram (definition) men. If this is the predominant practice in a country, then this work must be avoided. As for the statement that a fashion designer's destiny is Hellfire, it has no basis. The one deserving of Hellfire is he who designs for a woman who beautifies herself immodestly, not for one who adorns herself for her husband.