Git moved to a modified SHA-1 (let's call it SHA-1′) back in 2017 which prevents the particular attack described in SHAttered, so the SHA-1′ object names and signatures can be trusted for now. But the known collision attack on the underlying algorithm makes it more likely for SHA-1′ to be broken in the future too. Researchers could find variations or improvements of the weaknesses used in SHAttered to attack SHA-1′ as well. In other words, there are no known practical collision attacks, but there are known avenues of reasearch that could lead to practical collision or second-preimage attacks being discovered.

The idea, then, is to migrate all git history to a stronger hash before that happens. If we waited until an attack was found, it'd be too late; all git history would be suspect forever into the future (disregarding extensive auditing), even if a stronger hash was used retroactively. Full SHA-256 doesn't have known practical collision attacks, and it doesn't even have known research avenues likely to lead to practical collision attacks, so is considered more future-proof than SHA-1′.

This idea of future-proofing is common in cryptography. For example RSA-1024 keys are now considered practical to factorize (with a supercomputing cluster and a few months), but that's okay, because, foreseeing the possibility, "everyone" switched to RSA-2048 or stronger a decade ago. Now we're adopting new key algorithms to protect against possible quantum computing attacks in the future.