Is there a way to lock edge lengths of a mesh in edit mode? I want to edit a mesh without distorting it, so that only the angles between the edges change but not the length of the edges.

I basically need this to model realistic clothing where I model the 2d shape (that one would also cut out of real fabric) and then "sew" it together manually.

I already tried doing that by actually using sewing with a cloth simulation but that is not very efficient and does not leave a lot of freedom.
It sounds like using a curve modifier might be the most practical and fastest solution. This might distort the shape of the thread (if it is a mesh) slightly but should (as far as i know) preserve length.

But if it has to be in edit mode you could alternatively use the rotation tool when the rotation center is set at "active". Then, for example, if you select half of the string and the middle vertex is active you can change the angle between both halves of the string without changing any lengths.

Happy Blending! :)