When using FG7 on 1.20.4 and older without the renamer plugin, everything appears to work at first - it runs, stuff compiles, a jar gets built successfully - but the jar won't work in production.
It would be good if FG7 could show some kind of warning or problems report when the Minecraft.dependency or mappings version is one that needs remapping and the renamer plugin is missing, to avoid silently broken builds.