Do you mean admonitions? E.g. info, warning, etc? There’s precedent for that in commonly-used open source implementations, e.g. obsidian.md (which uses the same syntax, and started before). What semantics does it break? It’s designed to read well in plaintext and render nicely even if used in a renderer that doesn’t support admonitions, e.g.
[!NOTE] Information the user should notice even if skimming.
As opposed to other common markdownish implementations that use nonsensical plaintext which renders poorly in alternative renderers. Here’s a discussion on the topic in the CommonMark forums.