Yahoo withdrew IE11 support last year - they bombarded the interface on login with a popup saying IE11 will no longer be supported and the frequency kept increasing till finally if you hadn't upgraded your browser, you were defaulted to the basic version. (I took a screenshot of it at the time but its now archived)
BT says activity is you login once in 90 days and you should have seen this last year so there is no reason to say Yahoo mail is changing or has changed when they are doing the newer bt mail now.
The point the OP is making is that the basic version has changed, rather than the basic version has to be used. He (as do I for the very few occasions I use webmail to check for spam on a POP3 account) already uses the basic version from choice.
I was always using the basic version from choice, so I never got 'defaulted to the basic version', and I never saw any login messages about support for IE11 being withdrawn
what I did get was a strip along the top of the inbox telling me I was using the basic version because my browser didn't support the full-feature verson, and as I said before, that wasn't why I was using basic and I didn't want full-feature mail so it was irrelevant
the point is, as licquorice says. the basic version itself has changed, and because BT told me to use an option (which had been removed but BT support didn't know that, as gg30340 says they 'default' to BT Mail advice which doesn't always apply to BT Yahoo Mail) I thought I had a fault
the confusion is down to BT running 2 different mail systems with inconsistencies and not knowing the differences
so I have a simple choice - stick with violent purple basic mail, or run 2 browsers with everthing else on IE11 and full-feature mail on Edge and change the theme back to lovely serene blue! I am going to get the inbox ads either way now they have been introduced into basic mail, as with everything else I will stop even noticing them before too long
Or, as I keep advocating, stop using webmail altogether and use a mail client, a far far better solution.