What do you mean? This is perfectly modern. Material UI and minimal, outline style icon theme. That's all the rage with web devs nowadays. Amazing.
(Not that there's anything wrong with that in this case, that's the Android style after all. But personally I heavily dislike Material UI.)
I would say that this UI is ugly though. Spacing is all over the place, the icons don't look cohesive at all apart from the colors used (for example, rounded vs sharp corners), the yellowed paper looking background color, overuse of bold/italic/colored text (especially multiple of those at the same time), inconsistent display of the same thing (in one screenshot the mailbox name is displayed as "Gmail", in the other as "[Gmail]"). And so on.
But that doesn't mean it's "outdated", this would have been equally as bad 10 years ago.
(I just have a knee jerk reaction to people saying "outdated UI" because usually it's used as a justification to replacing perfectly well designed UI with a worse version just so that it follows contemporary design trends. Cf the Windows Settings app.)
The app uses Material UI, sure, but it's anything but minimalist. It feels the dev(s) tried to use and cram as much as possible from what's available from the design without really thinking of usability. Information density HAS to be lower when using a phone, since the screens as much smaller than a monitor and you don't have as much precision when using fingers to navigate around.
That's half the reason I like Thunderbird. Email hasn't changed for 20 years, and neither has Thunderbird's interface. I don't need shadows and 3D effects and stylised colours and buttons, I just want a white page with black text displaying the content of my incoming messages.
You seem to be a function over form person, and I'm a function and form person. Surely, email hasn't changed since ever and all I need to see it's contents is a white page with black text, but that doesn't mean everything else has too look bad or lacking meanwhile, specially on a phone.
The menus are definitely busy and confusing (there are many options), but once it's set up I've never been bothered by the UI.
I quite like how emails are shown OOTB in fact, with the right padding and day separation; I also use most buttons that are offered by default. So yeah, sane defaults.
Off the top of my head what I like:
you can search emails on the server! That's the one feature I couldn't forgo
it detects unsubscribe links
I remember having delays when using K9, with FairEmail not once. This could be fixed now
it cleverly shows you're connected to a VPN (which can be a problem with emails)
maybe too many options, but a lot are actually useful
Not trying to say it's better, to each their own. But it's great.
edit: I received an email at 06:19 in fairemail. it's now 06:56 and I just received it in thunderbird