I think you’ll have to be far more precise in your description of “certain functional restrictions”. Balancing an unverified user’s ability to meaningfully interact with the community without turning the feature into an abuser’s dream is going to be very difficult, and asking the Discourse team to do the work to thread that needle, in addition to the actual implementation work, is asking a lot, for a feature that I’m fairly sure will not see widespread adoption.
Also, I wonder if you’ve thought through the full implications of the “automatic cleanup”. By permanently deleting the account and all related data, you both have the problem that Jakke highlighted (a “temporary” user comes back a month later to find their contributions nuked), but the “holes” in conversations that are left by permanently deleting a user are significant (there’s a reason why users with non-trivial contributions are usually anonymised, rather than being deleted).