Compared to other Web Drive providers, Proton Drive lacks a feature where items whose 'Deleted Time' exceeds a certain threshold (i.e. 30 days) get automatically deleted by the system.
There are a number of reasons as to why this might be so. One possible reason could be that, due to data encryption on Proton's side, they cannot view certain metadata of the file, such as its 'Deleted Time'. This would then exclude any attempts by the server to perform housekeeping duties.

The solution is to move the deletion routine into the user clients instead. All the server has to do is track a new variable storing the number of days for trash retention, which the user would set to their preference. When a client app connects to the Drive, it would retrieve the stored value and then perform the deletion routine. To minimise server traffic, clients should only do the check once every 24 hours.

For browser clients, the user would have to click into the Trash folder and click on a specific button, which would manually initiate the deletion process. I'm sure some client-side JavaScript could easily do this securely.