You tell such users: "Your browser is too old, please update it" and direct them to the Chrome and Firefox websites.
With the exception of Safari, all major browsers support auto-updating, and anybody who intentionally uses an outdated version of Safari is a masochist with a deviant fetish for error messages.
Feature phones, smart-tv/watches etc. do not auto update. Old versions of android, iOS, Windows. Non-mainstream OS's running non-mainstream browsers. 1% of the market is still a lot of users.
And in the idealized fantasy world of Ayn Rand, any giant dinosaur enterprise corporation who insists on using antique outdated Jurrasic browsers deserves to be bitch-slapped by the invisible hand of the market into extinction for their outrageously dangerous security and privacy policies.
To bad the real world doesn't work the same as it does in self-indulgent libertarian porn...
The real world works exactly like that. It's called System Requirements. If you want to use a piece of software, you look at the requirements and if you don't meet the requirements then you can't install it.
All websites have system requirements, no matter how non-libertarian they seek to be, for example most websites will not work on IE6.
It is not a fantasy to inform users that they don't meet the requirements, it is actually the courteous thing to do. If more websites were bold enough to inform users of their outdatedness then we would not be having this conversation.
This is especially true for businesses. I've had a few cases where clients balked at browser support policies but when presented them with the cost multiplier it'd add, not to mention the cost of features / security, that clicked it into a calculation and suddenly it turned out that the “hard requirement” was actually one VP who refused to upgrade from IE5 Mac and they decided not to pay more for that.
I mean the real world is not like Atlas Shrugged, which is archetypical self-indulgent libertarian fantasy porn. The invisible hand isn't God setting things right and making life fair, it's rich assholes (who worship Ayn Rand and think they're God) rigging the system to cheat for themselves.
With the exception of Safari, all major browsers support auto-updating, and anybody who intentionally uses an outdated version of Safari is a masochist with a deviant fetish for error messages.