Filter by date is already a thing. But not exposed to users in simplified filter creator for few reasons:
1. Filters run once per email as soon as it is received. Which means when the filter is run automatically, the date will end up being `currentdate`.
2. Adding date to simplified interface would cause confusion, and make it not so simplified.
3. Power users who need such filtering can use sieve editor. Sieve is a programming language, and you can use

Proton's sieve documentation on date: https://proton.me/support/sieve-advanced-custom-filters#vacation
Official Sieve language documentation for date comparisons: https://datatracker.ietf.org/doc/html/rfc5260#section-4

A site note about the suggested use case: Dumping all the stuff into Archive folder after some time will surely turn Archive into Trash folder without the 30 day expiration. Archive folder is a weird folder, and probably shouldn't even exist.

I'm a heavy filters user. I have quite a few that are specifically for automatically moving low priority emails out of my Inbox and into folders I check less frequently. However, in many cases, it would be nice to give myself a few days to consider before the email in question is swept off to a folder where I may or may not decide to give it any further attention. Having the option to edit these existing filters by setting them to only take action if the email is older than a few days would be a huge improvement!

would be especially useful if we do this with all read emails, and have all unread emails go to a different folder. Adjacent to that would amazing to sort by unread and also by un-replied.

Strongly seconding this. Date filtering is absolutely critical for automated cleanup.

It should allow us to delete based on date or age & folder / filter.

I want to bulk-delete mail created before a specific date.

Its been more than four years. Has this request for feature been denied?

Are there any plans on this suggestion?
The feature is really useful.

Then use the Export app to export the Archive folder only contents.