I'm assuming this has more to do with Excel's behavior than it does Forms, but I just noticed that when attempting to open a Forms URL from within an Excel file it displays the "Browser not supported" page.

My default browser is Microsoft Edge (Chromium) and the page is showing in Edge, but for whatever reason it still seems to be triggering the code recently added to prevent Forms from opening in unsupported browsers.

My best guess is that something Excel is doing is making the site think it is being opened in IE, but it's hard to say for sure and I wanted to mention it to see if anyone else has seen this or if there's a way to prevent it.