For now, a workaround that a Remote Metadata Adapter could adopt to achieve the same is to connect back to Lightstreamer Server from the client interface.
In practice, the Remote Metadata Adapter will handle the equivalent of the notifyNewTables method for its SDK in a way similar to that shown above.
Then, it should connect to the instance of Lightstreamer Server related to it and send the unsubscription request on behalf of the client. The sessionId of the involved session, along with the subscription's "win index" are sent as parameters. SessionIds are random and complex enough to be quite impossible to guess, so no specific authentication is required to send such request.
The "win index", i.e. the subscription's unique identifier within the session, is provided to the Remote Metadata Adapter in the equivalent of the TableInfo object for its SDK.

The protocol to use to express the termination request should match the protocol currently used by the client for the underlying session; but actually there is no such costraint and any supported protocol can be used.
This allows us to use the simplest protocol available, which is the old text protocol (i.e. the SDK for Generic Clients up to build 1803, before the introduction of the TLCP protocol).
An unsubscription request can be expressed in the old text protocol as a simple HTTP request with a URL of this form:
/lightstreamer/control.txt?LS_session=<the session id>&LS_op=delete&LS_table=<the win index>
The reference for the syntax is available here (see paragraph 4.3.1).

This protocol can be used always with Server 7.1. With 7.0 it may not be available for Enterprise licenses which only include the Web and/or Node.js SDKs. With earlier versions, it is available only for Presto and Vivace editions.
For all other cases, you can resort to the old internal javascript protocol. There is no reference for this protocol, but the request syntax is very similar. An unsubscription request can be expressed in this form:
/lightstreamer/control.js?LS_session=<the session id>&LS_op=delete&LS_table=<the win index>