In this thread, e.g. <https://forum.nim-lang.org/t/8404#54212>
Old `develop` links current library folder in nimble pkgs folder so any change is automatically available for code elsewhere depending on it. Useful for checking if the fix you are doing for a library works for other pieces of code elsewhere. New develop requires to specify the dependency on the local folder where you want it to be used (in particular it would have to be some kind of package with a nimble file, it would not work for a free standing “script”). Alternative workflow would be to call again `nimble install` for every change you do to a library. I think maybe old develop behavior could be still supported as an option to `nimble install` (`—link-folder` or similar) with the important caveat that if package generates binaries this would be outdated (I think old nimble develop did not care to build binaries)
