Is showMessageRequest optional, or supported by all clients?

It's not clear to me if showMessageRequest is optional or to be supported by all clients. There is a showMessage field on Client Capabilities, but it appears to have only been added when support for additionalPropertiesSupport was added. This makes me think that showMessageRequest was already there (without any capabilities to say it's available).

However, the text in https://github.com/microsoft/language-server-protocol/issues/69#issuecomment-247768458 talks about when showMessageRequest was added, the showMessage notification was left in not to break existing clients, suggesting there may have been clients existing before showMessageRequest was added.

So is it safe for servers to assume showMessageRequest is always supported, and if not, should it be clearer from the ClientCapabilities.showMessage field that this indicates whether the server supports showMessageRequest at all?

Start by reviewing the ClientCapabilities.showMessage field and the linked language-server-protocol discussion and commit. Trace how the protocol describes showMessageRequest versus the showMessage notification, then clarify whether support is required and what the capability field means; done means the specification or issue has an unambiguous answer.