So if I want to continue sharing files between Mac computers I should never update any of them to APFS?
PS: is het possible to run 10.14 and stay on HFS+ ?
If you mean sharing files over the network, that should still work between different Macs regardless of the disk format.
If you mean being able to plug in a APFS formatted USB drive or connecting a Mac in target disk mode, then no. Earlier OSes won't be able to read the APFS formatting.