deprecate cache backend (#5382)

backend/cache/cache.go

@@ -339,8 +339,14 @@ func parseRootPath(path string) (string, error) {
 	return strings.Trim(path, "/"), nil
 }
 
+var warnDeprecated sync.Once
+
 // NewFs constructs an Fs from the path, container:path
 func NewFs(ctx context.Context, name, rootPath string, m configmap.Mapper) (fs.Fs, error) {
+	warnDeprecated.Do(func() {
+		fs.Logf(nil, "WARNING: Cache backend is deprecated and may be removed in future. Please use VFS instead.")
+	})
+
 	// Parse config into Options struct
 	opt := new(Options)
 	err := configstruct.Set(m, opt)