Emails containing tax information or mortgage information or key business information really need this capability. With "Proton Sieve" it can be very easy to forget a "keep" statement and have many messages end up with expirations from subsequent filters. My recommendation would be to add a "built-in" folder/label, such as "LOCKED" that would prevent any messages from being lost that were locked unless they were manually unlocked. Manually or programmaticaly locked messages could be moved to folders and have additional labels added or removed.