Not sure what you are on about.
Yes, it's important for developers to continue to update code to support newer operating systems.
But, it's also some of your responsibility to check that apps that you use are still current, and that your information is still accurate, as much as you can discover - (an app is listed on their web site as actually dropping support for El Capitan, for example)
Many apps simply show the oldest operating system supported as a base level, but the app continues to work on newer operating systems.
That will vary, depending on the developer. You would need to check for specific apps to see if the developer provides specific notes about current level of support, and compatibility with other, newer operating systems.
Many developers will have that information available at their download/support pages --- or may be responsive to direct contact about upgrades.
You did mention LibreOffice, so note that LibreOffice is updated frequently, and the current version (5.4.1.2) still supports OS X 10.8 and UP.
Libre Office states in their release notes that the next version will drop support for OS X 10.8 --- so you will need OS X 10.9 or higher
I use LibreOffice daily on my High Sierra beta system, with no problems.
With that in mind, I expect that you will not have any problems with LibreOffice, until they drop support for El Capitan. At this rate, you may still use whatever is the current LibreOffice for at least another 3 years --- at which time, you could likely continue to use that LibreOffice version, if you wish to continue using your old MacPro.