For those wanting the behaviour similar to that of Swagger 1.2, where it groups the routes in the SwaggerUI on the first part of the route - then this worked for me:
AppHost.Plugins.Add(new ServiceStack.Api.OpenApi.OpenApiFeature()
{
OperationFilter = (verb, operation) =>
{
var path = operation.Tags[0];
operation.Tags.Clear();
operation.Tags.Add(path.Split('/').SkipWhile(string.IsNullOrEmpty).First());
}
});