I would expect Developer ID certificates to all expire by the changeover, with the only point for continuing to ship Developer ID certificates would be so that new app builds can work on pre-Mojave OS releases. Or I suppose skipping any Apple-run scans of your software.
Notarization is an advanced version of this where Apple adds to the signature "we have scanned the app for vulnerabilities and will continue to do so"
How does code signing work in Windows (for normal win32 programs)? I know I don't have any problems running 15 year old programs, which I have to assume predate any type of code signing.