void dimension

I decided to hobble together a working proof of concept for this, so hopefully this is useful.

Assuming you already have the void dimension itself set up, there are 2 main elements needed:

• A feature that replaces bedrock
• A procedure to check the player's Y coordinate and teleport the player below a threshold

The feature can just use the same generation as an ore blob, examples of which can be found by clicking the "Feature templates" button on the left hand side. You can use any block you like, whether it be air itself or another block of your choosing. However, the distribution should be mostly, if not, entirely within the bedrock layer (-64 to -60). Image of the feature placement as describe:

And here's the placement in action:

The procedure is fairly simple, checking on each player tick whether their Y coordinate is below a certain threshold, and if so, teleports them to the desired dimension. Now note that without any further modifications, the player would die from the void's void, so you have to teleport the player up. But you'll die of fall damage if you're not given either Resistance 5, Slow Falling, or some other method of negating fall damage. Image of the described procedure:

This should allow the player to fall below the world and get teleported while any other entity takes normal void damage (thanks to the test chickens that were definitely not harmed).

It may be a bit scuffed, and there may be room for improvements and optimizations, but I cooked this up in roughly 10 minutes of creating and testing, and I hope that this is the solution you were looking for.