Referring to this technical article - https://support.esri.com/en/technical-article/000019443,
This was working previously before the June update but now my embeded content no longer displays, I either get the iframe as a link, or I try to edit the hosted feature layer and paste an iframe in, it is automatically deleted. I even tried this with just a youtube embed like the example, same thing.
Our team has experienced some issues with HTML overall in pop-ups seemingly since the June update. With Experience Builder (fixed as of two weeks ago), and also with Dashboards. Hopefully this will get worked out soon.
I've been having a similar issue. When you say that the iframe you post is automatically deleted - do you get a little notice saying Data Error at the top of the attribute table? That's what's been happening to me, as well as the embeds just showing the iframe code displayed instead of the embedded content.
I am having similar issues. I have a feature layer in AGO with Iframes that are displayed in the pop-up window. I made a few data updates to my feature layer in August editing the layer in Pro (no changes to teh iframes) and now my iframes only show-up as code in the AGO pop-up. I have even tried overwriting the original layer and republlishing the layer on AGO and have the same issue. In Pro the iframe is visible. See snapshots below.
Pop-up in ArcGIS pro
Pop-up in AGO
Still having the same issue - on the suggestion of a colleague I checked the Allow origins configuration in the AGO Organization Security section to whitelist the domain my embedded content was coming from - but it didn't resolve the problem.
Worth checking if you are attempting to embed content in popups, however. Maybe it will work for someone, and if it does let us know!
AGO > Organization > Settings > Security > Allow origins