CRM Online: Mandatory Upgrade

Hello,

I have a customer on CRM 2015. We haven't been able, as yet, to test our customisations in CRM 2016. I'm conscious that time is getting on and worried that CRM 2016 may become a mandatory upgrade sometime soon. I've looked for roadmap somewhere that would detail this date, but have not been able to find one. Does one exist?

Thanks.