Options for New Chunks in Old Worlds

Something that would be nice is the options "protect me from the void" or "let me use the void" for old worlds updated to 1.17. Naturally, being protected from the void means putting a bedrock wall, as is currently the case on the Bedrock Edition Beta, and the other option just leads to the void. People love using the nether roof, so why not let us use below the overworld floor?