I recently received an email from a user asking if ArcGIS Online web maps were embeddable in Squarespace. While I do not have a full Squarespace account, I did get a trial to check things out. While I'll add a caveat that I didn't explore everything thoroughly, or exercise this with a full account, I can say that with a trial account everything seems to work just fine. Here's an embedded map from the ArcGIS Living Atlas of the World.
The way I grabbed the embed code was from the web map Share workflow. Share > Embed In Website.
I tried a couple of the available Squarespace blocks; Embed and Code. Both seemed to work easily, without any issues.
Again, I can't say this is the definitive bottom line on embedding web maps in Squarespace, and did find something posted here on GeoNet that seemed to suggest there may have been an issue a long while ago. But I'm reasonably confident that everything should just work. I'll also admit that I have not tried embedding web apps using an iframe, but don't see any reason why results would be different.
If you've had experiences with web maps or apps in Squarespace, I'd welcome your comments, learning of any issues you've experienced, and seeing what you've done.