Thank you for your response.
'In his blog post, he lists clients that will break after the IdenTrust DST Root CA X3 expires. These include versions of macOS older than 10.12.1, Windows versions older than XP Service Pack 3, iOS versions older than iOS 10, OpenSSL versions less than and including 1.0.2, and Firefox versions older than 50.”
I use Mac OS 10.11.6
I wonder what I will lose updating to 10.12.1