Ed Neville said:

tp://ftp.debian.org [1] was on the deprecate list I thought. Maybe
they're moving things around in relation to ftp* mothballing?

It's still the one given in the backports instructions <https://backports.debian.org/Instructions/> and that doesn't work for the usual hosts. 

I hadn't remembered the backports-sloppy section but it'd be interesting to see if the packages that appeared are the ones in that.