I opened Sourcetree and it notified me that there was a new update.
I'm on version 4.2.4 and the update is for 4.2.5. However nothing appears in the Sourcetree logs folder.
Reading the forums I opened Console and saw some relevant lines that would indicate why the update would fail: "public key shipped with the update doesn't match".
DSA signature verification failed: Error Domain=Internal CSSM error Code=-2147415733 "Internal error #8001094b at VerifyTransform_block_invoke /AppleInternal/Library/BuildRoots/8ca92091-1d5a-11ee-a938-46d450270006/Library/Caches/com.apple.xbs/Sources/Security/OSX/libsecurity_transform/lib/SecSignVerifyTransform.c:594" UserInfo={NSDescription=Internal error #8001094b at VerifyTransform_block_invoke /AppleInternal/Library/BuildRoots/8ca92091-1d5a-11ee-a938-46d450270006/Library/Caches/com.apple.xbs/Sources/Security/OSX/libsecurity_transform/lib/SecSignVerifyTransform.c:594}
The update has a public (Ed)DSA key, but the public key shipped with the update doesn't match the signature. To prevent future problems, the update will be rejected.
Error: An error occurred while extracting the archive. Please try again later. The update is improperly signed. (URL (null))

Interestingly the release says 3 Oct 2023, but that's 5 days away. Anything to do?