What is the best way to assess payload restrictions?
As I recall it used to be clear if a route was limited to a certain number of seats but its hard to tell now, unless I'm missing something.
I can look at route performance, and see the usual number of seats sold and then the % of seats sold can infer the maximum available. Even the payload restricted box on managing routes doesn't tell you how restricted they are. When creating a route you do get some indication but that's expressly an estimate and varies from one plane to another. I have massive demand for an airport with a short runway and keep getting planes I think can handle it but they end up with unexpected payload restrictions
You can use the aircraft's Payload-Range chart (access from aircraft info page).
There should typically be a box that appears at route creation after you select your aircraft type that will tell you any limitations on each model of the type currently in your fleet. It doesn't help much if you are trying to plan well ahead on what might be the best plane for your situation but it will help prevent you from opening a route that won't work too well.