At least for Windows: if browser support is being considered, Firefox is the last browser that somewhat works (both IE and Chtome fails because of TLS 1.2) and it includes the ISRG root (because duh), and if an application still updates on XP there is a good chance that it uses OpenSSL or derivatives (which uses a different root set, likely Mozilla's).
Natively? Windows 7, assuming you have installed all updates before January. Microsoft can update the roots as they please (and historically issued updates up unto Windows 2000, so root updates are a demonstrable solved problem).
BSDs and Linuxes: (Usually) Uses Mozilla's trust list. Also updates separately from system updates, so unless you stick somehow with unsupported systems you already have this (and can be manually included into the trusted roots if you insist on using that outdated version).
macOS: bundled with the system updates (see caveat with Firefox independently managing roots). Here, I don't know what version is the oldest one with ISRG root certs.
As far as marketshare goes iOS 13 and 12 make up 94% of devices [2]. So I am guessing its an insignificant amount of actual users.
[1] https://support.apple.com/en-us/HT204132
[2] https://developer.apple.com/support/app-store/