About a decade ago (give or take), it <i>seemed</i> like WSDLs were incredibly popular for defining API accesses to network-enabled platforms<p>What happened to them / why do they [seem] to no longer exist?
I remember WSDL being a thing for "web services" that use XML but the world has gone towards using JSON in HTTP APIs.<p>I also remember WSDL as being part of a Microsoft-oriented stack that had (in my mind) somewhat awkward tools if you were working in ASP.NET (I did) but where you could really struggle if you were outside that world. I remember being tasked with developed a PHP API client for a service that used the WS-stack and figuring out how to get authentication to work in PHP (not well documented, I think I might have been the first.) I ran into even more problems after that and we just decided to switch to a competitive API that was easier to access.<p>Since then I have worked in Java (a lot like C#) where people often use Jackson or a similar library to talk JSON. In that case your "Schema" is usually a set of simple model objects. Except for ongoing hassles involving date and time processing it works great and is very easy to use. In the JSON world people learned how to get it done without a formal contract between the server and client. There certainly are schema languages for that but people don't tend to use them.
WCF services and SOA on Microsoft stack with WSDL was a thing in large enterprises Absa government. There may be a few services still running but please don’t make us go back there. For the most part I just remember them being overhead for what could be seen as remote procedure calls, they were partially or never services in the true sense at least from what I’ve seen.<p>Now there are microservices with rpc, rest or events. May be similar but less design time binding
The world moved away from SOAP web services utilizing XML to REST web services utilizing JSON. There's no "official" means for documenting REST/JSON web services, as the intent was to make them so easy to use that wouldn't be necessary, but if you do want to document them a lot of people use Swagger.