When a major change to the OS occurs, previous versions stop being supported. In some cases, then continue to work, but in others, they don't. Mojave made significant changes and required a new version. VMWare has to fund development required to address those OS changes, and that's paid for by new versions.
In this case, Fusion 8 was released in 2015 (paid), 8.5 in 2016 (free), 10 in 2017 (paid but many continue to use 8, unsupported), and 11 in 2018 (paid and required for Mojave support). Parallels has a similar history of versions and paid upgrades.
This year is unknown yet, but we'll get an indication in June at Apple's developer conference. If there's no major changes, then Fusion 11 should work (even if not supported), and we may even see an 'officially supported' version. If Apple does more plumbing changes, then it'll be a paid upgrade again.
Just a note, Apple also dropped support for Windows 7 bootcamp along the way, so if you're not on 10, it won't work regardless of the Fusion version. Windows 7 continues to work as a true virtual machine.