Is there any resource to explain all decisions made on WordPress?
No, there is none, and it’s rare for an open source project to have one, especially one of WordPress’ age.
For example,
<br>,<ol type="a">don’t work.
But they do work, and there is even code to explicitly allow them and enable them in the source code of WordPress:
<br>: https://github.com/WordPress/wordpress-develop/blob/6.2/src/wp-includes/kses.php#L110<ol type="a">: https://github.com/WordPress/wordpress-develop/blob/6.2/src/wp-includes/kses.php#L367-L369
If you are unable to use these, it is not because WordPress decided to disable them, WordPress allows them. Something else is causing your problems.
If you’re curious how it was decided what is in that list, the contents of that list are determined by the security and safety of the respective tags. Post content shouldn’t contain anything dangerous such as script tags or things that would allow those to happen, but blocks filters and shortcodes can be used to insert these safely so that embeds and interactive content are still possible. Trac tickets and the commit history are the closest to the decision record you seek, but they will not help you with your current predicament.