Slightly tangential, but Markdown does actually "kinda" support comments with a little bit of abuse of the link syntax. Almost every renderer will swallow the comment in the following example:

    > This is a quote of some kind

    [//]: # (This won't be in the rendered output.)

    Blah blah blah

Keypoints brings a lot of other things to the table than just comments, including an upcoming possibility of crosslinking between PDF docs [0], but I've wanted to add annotations often enough to small Markdown documents enough that I've memorised the above little trick. It even works with the original Markdown.pl implementation.

[0] https://keypoints.app/forums/viewtopic.php?f=2&p=788