The CustomizeBlockEvent isn't really useful as the blocks can just be set via configs and should remain world persistent.
It is highly unlikely that any mod would utilise this event (which would require a dependency on the RTG API) and it creates an environment in which another mod could corrupt the world by arbitrarily changing which blocks are used during chunk generation.

No UBC doesn't use that event anymore in 1.12.2.

Then we burn it to the ground!!! 🔥🔥🔥

IIRC, this event was added to allow UBC to 'do its thang' with RTG's boulders, so if UBC isn't using this event, then I don't see any harm in removing it. But either way, I don't think this needs to be sorted before we release for 1.12

Closed in d6b7a93