ProxyServices API has three methods that we no longer use in code and those can be removed. These methods were deprecated even in Weld 3 and Weld 4 so no need to carry them over to Weld 5.
Integrators might have their own implementations of this service and the only thing that might cause them issues is if their methods were annotated with @Override.
As for the implementations needing a change, we have our own DefaultWeldProxyServices in core repo.
And there is also WFLY-specific implementation which will have to be adapted.