I still only have iOS 16.3 and I've noticed mine are gone too.
I've also tried every suggestion in this thread but no luck (including contacting Apple). This is a really unacceptable thing to happen and they need to find a way to restore our hard-earned collections of places. We depend on them.
Incidentally, I just recently got an Android phone just to get acquainted with their apps for the sake of suggesting good ones I find to my clients, but having all of my guides vanish from Maps makes me want to activate it and stop using iPhones altogether from now on since Apple clearly can't get their act together in yet another way.