To the best of my knowledge Ubuntu LTS versions such as 18.04 have the same 5 years of support since Ubuntu v.12.04 whether you've installed a desktop version or a server version. The support schedule prior to 12.04 was 5 years for server and 3 for desktop. Of course any packages that you choose to install from PPA's you do so at your own risk.
So, no I don't see any reason to expect that your server will be downgraded to 3 years of support. Of course if you've purchased a support contract somewhere, it's always best to review the terms of any contract to insure compliance.
To obtain a list of packages that are reported to be supported you can issue the command:
To obtain a list of your packages that are reported to be unsupported you can issue the command:
Note that just because a package is listed as unsupported doesn't mean you won't be able to find someone to help you with it here, or on another Stack Exchange site.