Well, this could change at any time. Part of the point was to argue as if SHA1 was completely broken and I still feel that its the correct argument.
But from your chosen-prefix argument, having some object thats been around a while is a problem, because any viable attack needs to be fairly new, since git wont replace objects it already thinks it has. Maybe fresh-shallow-clone scenarios like GitHub actions, but that still always has a non-sha based authentication protection (in other words, actions never run on untrusted code and that trust is never based on signed artifacts but on source provenance)
But from your chosen-prefix argument, having some object thats been around a while is a problem, because any viable attack needs to be fairly new, since git wont replace objects it already thinks it has. Maybe fresh-shallow-clone scenarios like GitHub actions, but that still always has a non-sha based authentication protection (in other words, actions never run on untrusted code and that trust is never based on signed artifacts but on source provenance)