Review the list of obsolete types and see whether we can remove any of these in .NET 6

For .NET 6, we’d like to evaluate the APIs marked as obsolete and remove them if necessary. There’s ~130 APIs marked as obsolete that are split up based on project area in the table below. The goal is to have these APIs removed by preview1 so we have time to get feedback on our decision early on in the release.

What to do

• If it should be removed
  • Remove it
  • Update any samples/tests that use the obsolete API.
• If it should not be removed:
  • Add an inline comment explaining why and provide a timeline for removal if possible
  • Ensure that the Obsolete description provides information about alternative APIs or guidelines on what the user should do otherwise.
  • Provide an aka.ms link to the deprecation announcement.

Who is doing it

Area	Assigned	Done?
Azure AD	@HaoK, @JunTaoLuo, @javiercn	https://github.com/dotnet/aspnetcore/pull/27955
Hosting	@Tratcher, @halter73	https://github.com/dotnet/aspnetcore/issues/27529#issuecomment-748288221
Http	@Tratcher, @jkotalik , @halter73	https://github.com/dotnet/aspnetcore/pull/28890
Middleware	@Tratcher, @HaoK, @halter73	https://github.com/dotnet/aspnetcore/pull/28893
MVC	@pranavkm, @javiercn, @captainsafia	https://github.com/dotnet/aspnetcore/pull/28337
Razor	@ajaybhargavb, @captainsafia	https://github.com/dotnet/aspnetcore/pull/28706
Servers	@jkotalik, @halter73	#29275
SignalR	@BrennanConroy	https://github.com/dotnet/aspnetcore/pull/28100

Things to keep in mind @halter73 and @captainsafia took a look at all the APIs and made the following list of things to consider when evaluating whether or not an Obsoleted API should be removed.

• Outside of redundancy, is there another harm to having the API around? For example, it is not performant or not supported by other parts of the ecosystem.
• What’s the maintenance cost of keeping the API around? This includes things like the number of issues reported by users, whether there’s a lot of flaky tests associated with the API, etc.
• Does the Obsoleted API no longer work?
• How common is the API that is being obsoleted? Is it likely a new user will inadvertently use it and run into issues?
• Is it actually a shipped API?
• When was the API obsoleted? If it was obsoleted in .NET 5, keep it around to make upgrading between LTS releases, from 3.1 to 6, easier for devs.
• Is this API commonly used by other packages?

[javiercn]

We should consider adding aka.ms links to the obsolete message that point to a landing doc (the announcement?) and that suggest alternatives.

[Pilchie]

Also to keep in mind - if the API was first marked Obsolete in .NET 5.0, we might consider keeping it for .NET 6 to make it easier for people to upgrade from LTS to LTS (.NET Core 3.1 -> .NET 6.0).