After modifying /etc/apt/sources.list as explained on
http://snapshot.debian.org/ and doing

  apt-get -o Acquire::Check-Valid-Until=false update

I get among various messages:

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

which is completely unacceptable.

Control: forcemerge 763419 -1

snapshot.d.o provides read-only snapshots of the archive, it does not
modify any files.

If you would like to help fix this issue, please see the proposal in
#763419 for creating a dynamic overlay to modify and re-sign the
existing files.

I did write a prototype once, but haven't touched it for some time. For
example:

https://defi.43-1.org/debian-defi-archive/debian/dists/stretch-backports/InRelease

(It should also work for other things.)

The test key is available from
https://defi.43-1.org/defibrillator-test-key.asc

Ansgar