Thanks for the response. The concerns are privacy and security as well as the things users have in place to address those concerns.
As @hellcx9rv4 mentions, iframes can be a security hole, and as mentioned in article in the linked post, the types of site which frequently serve up embeds (Google, Meta, etc) should be considered untrustworthy, and increasingly are.
It’s reassuring to know that the devs are thinking about this, but concerned users will still be suspicious of the iframes on discourse sites just as they are on other sites, and they will be using the same mechanisms to address their concerns.
For instance, uBlock Origin has 6.4 million Firefox users and 10 million Chrome users according to their respective add-on / extension websites. While iframe blocking is not enabled out-of-the-box and I doubt it’s possible to know how many users have it enabled, these usage numbers indicate that a significant number of web users care about this issue. If the cross section of discourse users and uBO users blocking iframes (or iframe provider domains) is 0.1% of those numbers, that’s still 16,000 affected discourse users.