FTTC how is speed limit achieved for slower services?

ISPs usually offer faster and slower FTTC services. How is the service constrained to the lower speed where the line has a low enough attenuation to connect at a higher speed?

A basic software cap is apllied to limit the sync speed

It's capped at 55/40Mb the exact same way it's capped at 80Mb for those living right next to the cabinet. The sync speed is simply capped by software. That's why upgrading from say 40/10 to 80/20 can be done at the touch of a button, requiring no visit to the cabinet.