"Switch to the newest att.net mail" no longer working
I am an "AT&T Yahoo Mail" subscriber. My email address belongs to the Pacbell domain. I have AT&T DSL internet. I can manage my email either via mail.yahoo.com or mail.att.net, using a username and password that are managed via the AT&T website. Typically, my email is displayed in the "Newest att.net mail" GUI, including rich-text functionality, keyboard shortcuts, etc. About a week ago, a problem began to occur: When I would access my email in the normal way (typically via mail.yahoo.com), it would display in the Classic GUI instead of the "new" GUI, and I needed to click "Switch to the newest att.net mail" to see the "new AT&T" GUI. I tried this on several operating systems (Linux, Windows, Chrome OS) and with several different browsers (Firefox, Edge, Chrome, Chromium), but I could not get my email to open in the "new" format without clicking the "Switch" link.
A day or two ago, the problem worsened: Now, even when I click the "Switch to the newest att.net mail" link, it does not switch. The screen simply refreshes but remains in the Classic view. Again, this happens across multiple browsers and operating systems and with multiple Yahoo-AT&T Email accounts, both Pacbell and other domains. AT&T customer service said this must be a Yahoo problem (claiming that Yahoo is doing "maintenance on its mail servers"), and could not even give me a customer service number at Yahoo to call (let alone warm-transfer me or resolve the issue themselves). Yahoo (when I was actually able to speak with them) said this is an AT&T problem.
What is going on, and will it be resolved?