I’ve been wondering about this for a while. Here’s a proof of concept (note that it doesn’t solve the issue of removing scrollbars from iframes).
Add a script tag to the post that you’re embedding:
<script>
function sendHeight() {
const body = document.body,
html = document.documentElement;
const height = Math.max(body.scrollHeight, body.offsetHeight,
html.clientHeight, html.scrollHeight, html.offsetHeight);
window.parent.postMessage({
'iframeHeight': height,
'iframeId': 'zalgFrame' // Use a unique identifier if you have multiple iframes
}, '*'); // Consider specifying the parent domain for security
}
// Send initial height
window.onload = sendHeight;
// Optional: Update height on resize or other events
window.onresize = sendHeight;
</script>
I’m using the identifier "zalgFrame" in the script.
In your Discourse theme:
<script type="text/discourse-plugin" version="1.29.0">
let iframeHeight, iframeId;
window.addEventListener('message', (event) => {
if (event.origin !== "http://wp-discourse.test") return; // my test domain, update it to your domain or comment out
// gets the iframe height that's passed from `wp-discourse.test` and confirms that the iframeId matches the iframeID I set there
if (event.data.iframeHeight && event.data.iframeId === 'zalgFrame') {
// visit the Discourse page with the iframe with your console open
// you should see updated heights being sent from the parent site as you resize the window
console.log("we got an event:" + event.data.iframeHeight);
iframeHeight = event.data.iframeHeight;
iframeId = event.data.iframeId;
}
}, false);
</script>
In a Discourse post:
<div data-iframe-test-one>
<iframe src="http://wp-discourse.test/zalg_iframe/this-is-a-test-this-is-only-a-test/" width="100%" height="1659"></iframe>
</div>
So it’s possible to get the actual height of the rendered iframe from the parent window.
I don’t know how to get the height from the data from the event listener into a call to api.decorateCookedElement though. I’m not certain that would even work to remove the vertical scroll bar from long iframes. If I try hard coding a large height (1600px) into the iframe element, I’m still ending up with a scroll bar.
Edit: for the sake of completeness:
<script type="text/discourse-plugin" version="1.29.0">
api.decorateCookedElement(
(e) => {
let iframeHeight, iframeId;
function handleMessage(event) {
if (event.origin !== "http://wp-discourse.test") return;
if (event.data.iframeHeight && event.data.iframeId === "zalgFrame") {
iframeHeight = event.data.iframeHeight;
iframeId = event.data.iframeId;
// based on the assumption that there will only be 1 iframe wrapped in the data-zalgFram div
let iframe = e.querySelector("[data-zalgFrame] iframe");
if (iframe) {
iframe.style.height = `${iframeHeight}px`;
}
// after setting the actual rendered height of the iframe
// remove the event listener
window.removeEventListener("message", handleMessage, false);
}
}
window.addEventListener("message", handleMessage, false);
},
{ id: "component-id", onlyStream: true }
);
</script>
For anything over ~1000px in height, there doesn’t seem to be any way of avoiding a scroll bar being added by Discourse, so I’m not recommending the approach.
I think the answer to the OP is that it’s kind of possible, but probably doesn’t accomplish much. (Except that I learned about the window.postMessage() method 
