Given that Apple appears to have stopped selling the regular (client) edition of Snow Leopard via their online store shortly before Catalina was released (archive.org shows the purchase page was there on 21 September but was redirected to the refurbished sale page by 27 September), they may have also stopped selling the server edition.
If willa couldn't buy one around 2017/2018, Apple may have been selling off remaining stock until none were left, which could have varied by region.
My three copies from Apple in late 2015 were ordered from the New Zealand store (run by Apple Australia) and must have been in stock in Australia at that point, as they were delivered two business days later.