In reply to Justin Tadlock.

Yes, every theme developer uses a different approach for changelogs. I used to include a changelog.txt in all my themes.

I personally switched to use the readme.txt for changelogs for the following reasons:

1) There is already a valid standard for the readme.txt for WordPress plugins.

2) Therefore there exist already classes to parse the readme.txt which I could utilize for the plugin.

3) Otto said on the Theme Changelog proposal that readme.txt will be the way to go: https://make.wordpress.org/themes/2015/04/26/changelog-proposal/#comment-41266

So when theme changelogs come to WordPress Core they most likely will be using readme.txt.

As result I think it is better to get theme authors to include a valid readme.txt in their themes, rather than support all different types of files and formats.