So according to Marius in the last Q&A, having a newer version of Halium allows us to benefit from newer drivers and config file for a better support of VoLTE on the given chipset.
So having a newer halium version will help improve the quality and the ease of use.

So it's not a yes/no answer, it will depend on the device and the maintainer of the port.

For a specific device, the porter can try to integrate the plugin hoping that the config files and radio drivers were already good enough.
VoLTE needs a new port for the devices, so the porters might want to switch halium base in order to get the more recent drivers and config files hoping not many regressions.
As an example, Alfred is considering switching halium base for the Pixel3a in order to get better drivers for the modem, GPS, ...

So a recent halium version makes things easier but it's not a requirement.
But there is no hard requirement like kernel version to allow systemd during the move from xenial to focal.

I hope this clarifies things.