That's correct. Sorry for the trouble. For what it's worth, most devices from the last 10 years should be able to upgrade to 10.12. See this post for details: Certificates are not trusted on Chrome and Safari on old iMac with El Capitan - #24 by jsha. If there are specific problems your visitors are having with upgrades, I would be curious to hear about them (though ultimately they will need to get help from Apple Support).
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