That’s intentional, complex HTML support is only done via the “passthrough” HTML block, as you saw. Writing HTML, although partially supported in elements like <kbd>, is not expected to be fully supported in the new editor in a visual way, hence the HTML editor.
That said, if we consider this HTML.
<hr>
<H2 style="color:red;">Wiki article under construction</h2>
<p> Please allow the creator to work on this as they are able. You are free to offer help or advice by replying, but please do not edit it without permission.</p>
<hr>
The style gets stripped by the sanitizer, why not write it like this:
---
## Wiki article under construction
Please allow the creator to work on this as they are able. You are free to offer help or advice by replying, but please do not edit it without permission.
---