I found the /etc/apt/sources.list.d/lexicographic-order.list to /etc/apt/sources.list.d/lexicographic-order.sources (Debian RFC822 control data format) migration well-planned: the old format will not stop working before 2029, just newly installed or upgraded systems will not default to it any more. Unless an admin specifically deployed no longer compatible files to a newer release, without even checking how the folder looks there, this slow changeover won't break much. Which is the sort of planning I value Debian for, and hope for whatever cleanup the mirrorlist plus http-redirect bit eventually gets. (There is additionally the somewhat related pdiffs feature, which could suffer from the same category of time-of-check/time-of-use bugs - but that one already is configurable per-repository and less seriously harmful if disabled on a supporting source.)