What if I have a Server that can not Upgrade to 10.15 or later?

If you’re limited to macOS 10.14 Mojave, this is presumably a Mac Pro 2010 or 2012 with Metal graphics.

You can serve files from macOS, no need for Server for that.

Server itself ended at High Sierra, transitioning into an MDM package with Mojave.

Changes introduced in macOS Server 5.7.1 - Apple Support

There are folks with add-ons for macOS that will allow server-oriented functions with Mojave and yet newer releases.

As for file sharing... FreeNAS might well load and boot on this Mac Pro.

Alternatives here include Mac mini (which will be far more efficient than this Mac Pro), or a NAS box such as those from Synology. I’m in the midst of deploying a 24 TB configuration (6 GB / 12 TB RAID-1) Synology NAS for ~US$1K, and the NAS box is the size of a toaster. Synology can also host Time Machine backups, as can some other NAS boxes.

I like and use 2012-era Mac Pro servers, but those old Xeon boxes are all getting... old.

There are two ways:

1) "Torrent Edition"

2) Buy it in mac appstore with Catalina (no need to download after purchase). Then from target Mac download it. App store gives you latest Mojave-compatible version.

I think it will be 5.7-5.9. For El Capitan latest is 5.2

The number 2 is most comfortable because you'll be able to use older Macs as servers too with latest compatible versions

You might consider upgrading your server to macOS Sierra (10.12.6) and run macOS Server 5.3.1. Or upgrade to 10.14.6 and run Mojave's macOS Server which no longer has several features found in 5.3.1 and below.