Have you actually got any evidence of that? I’m not a plusnet customer, I really don’t care but it’s unhelpful for other people who may come into the thread who don’t understand how FTTC works.
The line training is done BT openreach’s cab, it’s got nothing to do with plusnet. Your line speed can change over time, e.g. cross talk gets introduced as the cabinet fills up and the speed drops for everyone accordingly.
Your line may have been good for 74mbs at one point but that doesn’t mean it is now. Line quality is dynamic and is absolutely impacted by external influences.
You might see it start to go up again when FTTP gets enabled and the cabinet starts emptying back out. I gained 10mb when they did some engineering work to lay the fibre for FTTP in my road. Who knows what they did, they might have replaced something or just moved some wires around in the ducts but all we know is that me and a few neighbours now get more speed, happy days.
To reiterate Plusnet, Talk Talk, Sky, BT, Zen etc. have zero control over the rate your modem syncs with the cabinet.