That's not new. The range of "acceptable" throughput speeds has always been very wide. That applies to the throughput speeds, not sync speed.
Actually it's been much lower in the past. In the WBC FTTC handbook issue 8, the minimum was 12 Mbps for lines with sync speeds 15 Mbps or more, otherwise 70% of the profile rate. Current documentation is not publicly available.
I believe BTWholesale offer an elevated service with a higher minimum acceptable speed (60 Mbps instead of 40 Mbps) for an additional cost, but I don't know which ISPs if any sell anything using it.
And yet people expect, and Plusnet seem to promise, that people will get 100% of their line speed at all times.
For the sync speed, Plusnet seem to operate by comparing it to your estimated speeds, from the A range, even though they do self-install.