Most packages that are still under active development now use dune, which, unlike oasis, ocamlbuild and other solutions, offers good Windows support. A special repository for Windows users is therefore becoming increasingly superfluous. In addition, there is now esy as an alternative, which also supports Windows.
To ease the transition, I will create a special branch in the near
future, which will only contain the compiler packages of the old
Windows repository, and can be used in addition to the normal upstream
repository (opam repo add ...). However, this branch will not be
maintained.
depext-cygwinports unfortunately became increasingly useless, as the main maintainer of the corresponding packages stopped his activities more than a year ago. Hopefully someone else will develop a suitable alternative; many packages rely on current third-party libraries like gmp.