Some may not be aware that BT still had some broadband packages which were data limited - usually 15GB per month.
E.g. the discounted BT basic+broadband for people on means tested benefits.
And some older broadband products which some might still be on.
They have now made all packages have unlimited data.
The change on the account usage tracker appears to have happened in the last day or so. Interestingly the announcement says this is a permanent change. Also it is back dated to February.
https://www.bt.com/coronavirus