Git commits are identified by a hash of their entire contents[1]. The way hashes work, if you change even one bit, the hash becomes completely different. Every commit contains the email address of the committer and the hash of the parent commit. If the email address in even one commit is changed or removed, that changes its hash, which in turn requires you to update its children, changing their hashes etc. So, updating a commit from n years ago requires you to update all commits that have been made since. By default, git will refuse to pull from such an updated repository, as commits are considered immutable once pushed.

[1] In practice, it's a bit more complicated. Merkle trees are involved, so it's hashes of hashes of hashes instead of hashing a multi-gigabyte blob on each commit, but that's a performance optimization that doesn't affect semantics much.