Collapse OS is distributed as a source tarball which is updated regularly. This tarball contains a change history in the form of RCS metadata. The high-level changelog can be peeked at directly here.
Collapse OS used to be published as a git repository. An archive of this repository is available here. Note that this repository also contains the first incarnation of Collapse OS entirely in z80 assembly. Checkout the "z80asm" branch to access it.