It's hard to estimate IE6 usage, it's a 17 years old browser and the last version was in Windows XP (last version in 2008) and Windows Server 2003 (last version 2007). So it's on board of computers that are at least 10 years old.
Three year old browsers are basically all evergreen browsers I think (that auto update).
It also depends on what kind of app we're talking about.
Is there a way you can find out, for each major browser, which versions of SSL they do not support? On Wikipedia there's some info en.wikipedia.org/wiki/Transport_La...
You might also install a few browsers with a VM and test websites like cc.dcsec.uni-hannover.de/ or ssllabs.com/ssltest/viewMyClient.html
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