As you know Microsoft bought Xamarin and, along with it, is now responsible for the ongoing development of Mono. In addition, they will be releasing .NET 5 in November, which is a cross-platform successor to the .NET Framework. They are saying that ASP.NET Core will be the only ASP.NET Framework and they are dropping support for Web Forms.
When this happens, will the Mono framework drop Web Forms too, or will they continue to develop?
When this happens, will the Mono framework drop Web Forms too, or will they continue to develop?
The short answer is yes, they will drop Web Forms too. You should consider Web Forms deprecated—both in terms of .NET 5 and the forthcoming versions of the Mono runtime. You will continue to be able to run Web Forms on legacy version of Mono and .NET Framework—at least insofar as you're able to today—but this support won't be carried into future versions.
To better understand this, it's useful to understand the relationship between .NET 5, the CoreBCL, the CoreCLR, and Mono. This has become a bit fuzzy since the CoreCLR now works across multiple platforms, thus overlapping with much of the territory previously handled by Mono.
At a high level, .NET 5 will utilize a newly unified .NET Core Base Class Library (or .NET Core BCL). That BCL will replace many of the class libraries that were previously handled by the various .NET implementations, including Mono. Mono, however, will continue to be used as the runtime for .NET 5 in environments where size is a higher priority than performance, such as mobile and WebAssembly. By contrast, the faster (but much larger) CoreCLR will (continue to) be used in other other environments, such as desktop and web server applications. The .NET Core BCL will operate on top of these runtimes.
In other words, in .NET 5, there won't be separate class libraries for Mono, as there is today. Everything will use the same base class library. And, on top of that, you'll have additional class libraries for e.g. ASP.NET, WPF, Xamarin, &c. That will be completely independent of what runtime they're using. As a result, since ASP.NET Core 5 won't support Web Forms, that means that Mono won't either, since they'll now be using the exact same class libraries.
There's an excellent interview with Miguel de Icaza on InfoQ that is worth reading for better understanding the direction of the Mono runtime and how it relates to .NET 5. (Miguel is the original author of Mono, co-founder of Xamarin, and continues to direct Mono development.) It doesn't talk about Web Forms specifically, but does a good job of clarifying the relationship between Mono and .NET 5.
An alternative answer. "When this happens, will the Mono framework drop Web Forms too, or will they continue to develop?" If Mono is to be abandoned, then your question might become completely invalid.
Roll back to 200x. Mono did initially support ASP.NET WebForms, but that was the time that ASP.NET 1.x/2.x/3.x was simple enough to clone, and the Mono team had little to do on other parts. Personally I consider that was the prime time of ASP.NET on Mono/Linux. However, when ASP.NET 4.x came with many important changes (async model and so on), gaps started to appear and many never were filled up (as the team shifted their focus on mobile platforms).
I don't know how you are using WebForms apps on Mono today, but if you were a serious user you probably hit tons of issues already and knew how difficult the situation was. So you should not really consider Mono/Linux a production level environment to run your WebForms apps, and you should go back to .NET Framework/Windows, or completely migrate to ASP.NET Core.
The rise of Xamarin and the acquisition by Microsoft have heavily changed the Mono landscape, so everything used by Xamarin products (Mono CLR and BCL) keeps updating, and other stuffs (including MonoDevelop, GTK#, XSP, and ASP.NET WebForms) are left behind. Since only Mono CLR is carred over to .NET 5, that can be seen as the end of Mono.
Of course, an open source project won't die easily. For example, since February 2020 MonoDevelop has been discontinued, but there has been a group of people trying to revive it (including me). But how far Mono can go is unclear and unreliable.
Miguel is currently optimistic on the future, but not everyone thinks the same.
