This issue was originally raised at https://gitter.im/solid/node-solid-server?at=59f0b46501110b7231fd132e with some interest in its use:

In order for applications to respect a user's privacy (to some extent [*]) an agent's profile description can signal their preferred proxy endpoint. Applications upon discovering this property (eg solid:proxyUrl or solid:preferredProxy ?) can use it where applicable. When this piece of information in not available, applications would typically use their built-in proxy endpoint as default. Hence, once the agent's preferred proxy endpoint is known, cool applications can override the default (eg dokieli/dokieli#230 )

solid:preferredProxy
  a rdf:Property ;
  rdfs:label "preferred proxy" ;
  rdfs:comment """An agent's preferred proxy endpoint for consuming applications to use. The value of the property is intended to be used as the base URL for the request eg. the preferred proxy as literal: `https://example.org/proxy?uri=` would be used to request `https://example.net/profile/card#me` as `https://example.org/proxy?uri=https://example.net/profile/card%23me`"""@en ;
  rdfs:domain foaf:Agent ;
  rdfs:range rdfs:Literal
  .

domain and range could be omitted in the vocabulary definition document. Range is a literal because we want the application to use that as the base URL for the intended request.

Example profile statement:

<http://csarven.ca/#i>
  solid:preferredProxy "https://example.org/proxy?uri=" .

I could not find an existing vocabulary that would indicate this, so I'm raising it here.

[*] Obviously applications do not have to care about this nor should agents rely on the application to use their preferred proxy endpoint. The bottom line is of course the level of trust an agent puts on the application that they use - which could very well mean that the application will use their built-in proxy endpoint for certain operations.