Once T334842: Replace usage of VirtualRESTServiceClient in Math extension is complete, investigate callers/consumers of getServiceClient() and see how it's being used. Attempt to see if you can replace getServiceClient() with getMultiHttpClient() and call run directly.
If everything goes well, this should be a direct replacement of getServiceClient() since MultiHttpClient provides a ::run() method and a ::runMulti() method, use them in the appropriate places and test locally (for example with 1 end point), to see if it works correctly.
NOTE: perform any minor / major refactoring to ensure that functionality is retained before and after the changes.
NOTE: Also note that MathRestbaseInterface::getUrl() needs to be compatible with what consumes it (about 5 usage with code search). If there are any changes to be made to make it compatible with the new client, fix it.