Are new macOS versions retained in the recovery partition? If a Mac is reinstalled by booting in to the recovery partition,
and the Mac came with El Capitan but was upgraded to Sierra via wifi,
what os will the Mac have after reinstalling via recover mode?
Found the answer to my own question:

The simple answer is, whenever you perform a major OS X upgrade, the Recovery HD partition is also upgraded to the same version of OS X. So, an upgrade from Lion to Mountain Lion will result in a Recovery HD linked to OS X Mountain Lion.

source: https://www.lifewire.com/identify-os-x-version-on-recovery-hd-partition-2259968