As a legacy version, 5.6.3 has developed several significant maintenance challenges for those still running it:
Does 5.6.3 delete deprecated services? - Apple Support Community macOS Server 5.6.3
Compatible with Macs capable of running High Sierra, including Late 2009 MacBooks and iMacs, and Mid 2010 Mac minis, Pros, and MacBook Pros. As a legacy version, 5
(Mail, DNS, DHCP, VPN, Websites, Wiki, Calendar, Contacts) As a legacy version