They've actually been already contacting people who use older programs via e-mails. A few industry artists I follow and even a few of my artist friends have been getting these warning e-mails.
Some people don't want to upgrade because they removed features that they use in the new version, so they need the old version.
Some people have not updated because the newer version of the program doesn't run in their windows OS, so they very literally can't update.
Another fun thing to note is a good amount of these people need these programs for school or work and would rather be using other programs like CSP, but work or school will not allow them.