Given that I have a dedicated public IPv4 address for a web server (Ubuntu).
What are the real disadvantages of disabling IPv6 for our users?
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By disabling IPv6 you - obviously - lock out any v6 traffic. Users behind NAT routers may be sharing the same IPv4 address - in case you want to be able to tell them apart.
This doesn't only apply to home / company users behind an on-premises firewall, it also applies to various other scenarios, e.g. Internet access providers that have run out of v4 addresses and put their customers behind private/public SNAT but also to Dual-Stack Lite customers that are IPv6 only and use CG-NAT.
If you think you don't 'need' IPv6: the Internet is currently in transition from v4 to v6 (for a couple of years) and every service should be "IPv6-ready" or rather IPv6-compatible. Disabling it is not a good plan for the future.