https://wccftech.com/rip-watchos-5-is-not-available-for-the-original-series-0-apple-watch/

Well, it has happened, and was going to eventually. watchOS 5 will not be available for the original Apple Watch; this also includes the insanely expensive Edition models from that era.

watchOS 5 packs a lot of cool features, and you can learn all about them here. It has so many features in fact that they might not even work on older hardware. Which hardware are we talking about? Well, the original Apple Watch to be more precise. If you have a Series 1, Series 2 or Series 3 Watch then you will receive the update later this year in the fall.