Source: https://github.com/matrix-org/synapse/blob/v1.59.0/docs/upgrade.md#deprecation-of-the-synapseappappservice-and-synapseappuser_dir-worker-application-types
As an alternative, we should probably find a way to run one or a few
more generic workers (which will handle appservice and user_dir stuff) and
update `homeserver.yaml` so that it would point to the name of these workers using
`notify_appservices_from_worker` and `update_user_directory_from_worker` options.
For now, this solves the deprecation, so we can have a peace of mind
going forward.
We're force-setting these worker counts to 0, so that we can clean up
existing homeservers which use these worker types. In the future, these
options will either be removed or repurposed (so that they transparently
create more generic workers that handle user_dir/appservice loads).
Slavi Pantaleev