Where has the option to display only unread messages gone from Mac Mail in classic layout?

Thanks, but I'm using "classic layout" and I don't see that button - that was my point.

And I have no difficulty selecting my "smart" mailbox that shows unread items. The issue is that if I switch between that mailbox and my inbox then Mail clears its search. So that makes it a poor replacement for the filter toggle, which des work in non-classic layout.

Thanks. But I did say I had already tried that solution and didn't think much of it. It's better than nothing, but I am used to being able to toggle between all messages or just the unread ones within the results of a search. Using a smart mailbox to select unread messages does not restore that functionality.

Well I can't afford to move to Catalina for quite a while yet (yes, it will involve spending real money replacing software and hardware it won't tolerate).

Thanks for the interesting information about what for me is a future system, but that doesn't go anywhere near answering my question. Or are you saying that the facility I've lost really is gone as part of the long term decline in the UI quality of the MacOS Mail app. I see from your other discussion that at least see people are unimpressed by the Use Column Layout in Catalina. Maybe the best suggestions really is to look for a better third party Mail app.