(As ZIP Editor.) I’ve assigned ZIP number 320 for a specification based on hanh’s proposal: ZIP 320: Defining an Address Type to which funds can only be sent from Transparent Addresses
(Taking off ZIP Editor hat.) @nuttycom and I will work on writing that up. We are currently intending to use a Unified Address Receiver type rather than a new Bech32m HRP. This has the advantage that it will be possible to associate an expiration height or time with the address, as specified in ZIP 316: MUST-understand Metadata Items and Address Expiration Metadata by nuttycom · Pull Request #759 · zcash/zips · GitHub.
Edit 2024-01-15: the resulting ZIP 320 has been published (as a draft) and describes both the “new Bech32m HRP” and “new Unified Address Receiver” approaches.