Interesting Reply Christopher,
IT I agree it is up to client / user to upgrade or not, but tell me when to don't delay and prompt to pay for your new upstage protection, and your customers keeps asking we want to stay on modern technology (SM16 No longer supports Video Conferencing) I think it is natural to be hunting after upgrade (of-course a working upgrade) and if it is not there yet, having wish to obtain it certain.
In My Cause I will elaborate more.
Since the Release of Outlook 2019 in Windows & Office 365 (ON Mac OSx Mojave) SM16 (Ver 16.3.6989) Auto Discover are not Functioning correctly along with MS Outlook 2019/ MS Office 365 Outlook in Mac OS Mojave).
IF it works in Exchange Environment. Then it is not working in IMAP (Talking about Auto-Discover) and when I open up ticket I am advised to upgrade to SM17 because Microsoft have made significant changes in their email clients recently.
and if I want to upgrade to SM17, then there is limitation about Plesk. I am stuck like petty between the Burger bun.
May be you have better idea !!