The old To/Cc design was really bad. If you have more than three addresses, then these three lines scroll. The only indication that there are more than three addresses is a tiny scrollbar all the way to the right. I recently sent an e-mail to more people than intended when editing an old message as new, because it had more To-addresses that weren't visible and I didn't notice the scrollbar.
The worst bit is that you can't even expand that part of the window to see more addresses at once! Need to include the whole team on this mail? Prepare to squint...
Now it needs some visual indication of a complete e-mail address in the To: line. Like a rounded rectangle 1px border around a complete e-mail address and some kind of hint, that you can enter more than one address.
There is a visual indication of a complete email addresses: each correct/complete email address is wrapped in a "pill", a light gray one (see OP screenshot). An incomplete email will similarly get wrapped into a dark red pill.
Your comment maybe indicates the gray is too subtle, or invisible to users with bad eyesight.
I'm very happy that it's being updated.