According to the Biome API documentation, OverworldBiomes.setRiverBiome(BiomeKeys.MOUNTAINS, null) should remove rivers for the selected biome (in our case, it's MOUNTAINS), however the game crashes: https://pastebin.com/fbyy6704

Maybe this is the reason

Makes sure that the given registry key is mapped in {@link BuiltinBiomes}

The ensureIdMapping looks like something added in 1.16 to automatically fix old mods when the biome registration mechanism changed (i.e. when worldgen got added to datapacks).

It shouldn't be applying this fix to null data.

The only workaround I can see, is to use the internal method that bypasses the validation and the data fix.

net.fabricmc.fabric.impl.biome.InternalBiomeData.getOverworldRivers().put(BiomeKeys.MOUNTAIN, null);