I assume you have some sort of "old browser compatible" marketing page, right?
I think your MARKETING has to be available on even older browsers (in at least some limited way - like a single landing page. And you can also measure how many people land there.
Our signup page supports older browsers, but our app does not. So:
- We can measure how many truly interested customers are on older browsers.
- We get their login o we can track how many then just use an appropriate browser.
So, I have a funnel in Mixpanel and I can see that only 2.33% of our users Login..unsupported.. and then of those 70% eventually login successfully. So we are losing about .33% of users due to browser incompatibility. Not ideal, but almost insignificant. And that problem will get smaller over time.