Git 2.45 is significant in this version having initial SHA1 and SHA256 interoperability for repositories . Git repositories with both SHA-1 and SHA-256 hash algorithm use can finally co-exist. Git has long been working on supporting the more secure SHA256 hashes rather than SHA1.
The other big aspect of Git 2.45 is having initial reftable support. Reftable is a new reference storage back-end for Git. Git's Reftable aims for faster performance with an entirely new format for storing Git references. With Git 2.45 new repositories can be initialized with the "--ref-format=reftable" argument for making use of it.
The lengthy list of changes and fixes to find overall with Git 2.45 can be found via the mailing list announcement . There's also a post on the GitHub Blog that covers the key v2.45 highlights in more detail.