chore: deprecate Gateway.PathPrefixes

Context:
#7702 (comment)

cmd/ipfs/daemon.go

@@ -319,9 +319,9 @@ func daemonFunc(req *cmds.Request, re cmds.ResponseEmitter, env cmds.Environment
 
 	// Start assembling node config
 	ncfg := &core.BuildCfg{
-		Repo:                        repo,
-		Permanent:                   true, // It is temporary way to signify that node is permanent
-		Online:                      !offline,
+		Repo:      repo,
+		Permanent: true, // It is temporary way to signify that node is permanent
+		Online:    !offline,
 		DisableEncryptedConnections: unencrypted,
 		ExtraOpts: map[string]bool{
 			"pubsub": pubsub,
@@ -677,6 +677,10 @@ func serveHTTPGateway(req *cmds.Request, cctx *oldcmds.Context) (<-chan error, e
 		opts = append(opts, corehttp.RedirectOption("", cfg.Gateway.RootRedirect))
 	}
 
+	if len(cfg.Gateway.PathPrefixes) > 0 {
+		log.Error("Support for X-Ipfs-Gateway-Prefix and Gateway.PathPrefixes is deprecated and will be removed in the next release: https://github.com/ipfs/go-ipfs/issues/7702")
+	}
+
 	node, err := cctx.ConstructNode()
 	if err != nil {
 		return nil, fmt.Errorf("serveHTTPGateway: ConstructNode() failed: %s", err)

core/corehttp/gateway_handler.go

@@ -159,6 +159,7 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 	// If the gateway is behind a reverse proxy and mounted at a sub-path,
 	// the prefix header can be set to signal this sub-path.
 	// It will be prepended to links in directory listings and the index.html redirect.
+	// TODO: this feature is deprecated and will be removed  (https://github.com/ipfs/go-ipfs/issues/7702)
 	prefix := ""
 	if prfx := r.Header.Get("X-Ipfs-Gateway-Prefix"); len(prfx) > 0 {
 		for _, p := range i.config.PathPrefixes {