The following message was sent to the dev-addons mailing list on 19-April-2019:
The following WebExtensions API are being deprecated:
The proxy.register() API use a proprietary PAC script because, at the time it was implemented, the proxy filter system was synchronous. It does not implement the standard PAC API, so normal PAC scripts (typically found in enterprise environments) do not work with this API. Now that the asynchronous proxy.onRequest() API is available, as well as the proxy.settings() API for controlling global Firefox proxy settings, the original API are being deprecated in order to decrease the cost of maintenance for the WebExtensions API.
The proxy.onProxyError() API was replaced by the proxy.onError() API which has been available since Firefox 60. The MDN documentation was updated to reflect this, but the API was never actually removed from Firefox.
These three API will be deprecated with the release of Firefox 71, currently scheduled for December 2019.
Developers using any of the three deprecated API in their extension should migrate to a supported API as soon as possible.
The status of the deprecation process can be followed on bugzilla .
Product Manager, Firefox WebExtensions