Signal client uses SQLcipher database to store conversation messages. Since SQLcipher
is essentially SQLite with encryption, it inherits SQLite features.

To update the database, SQLite (and consequently SQLcipher) uses Write-Ahead Logging.
All transactions are written to a Write-Ahead Log file, which is then merged to the
main database file when a certain number of pages are written to the log file, the
database is manually checkpointed, or the database file is reopened.

This results in an unexpected situation when messages are deleted in Signal, either
manually or by a timed deletion. The deletions from the database are recorded to the
Write-Ahead Log file, but they are not actually deleted from the database file until
the Write-Ahead Log is committed back to the main database.

By default, the Write-Ahead Log has a threshold size of 1000 pages. Depending on how
busy your Signal app is, this transaction limit could take several days to reach. This
means that the messages deleted in Signal will linger on disk far longer than expected.
The database file with the supposedly deleted messages can easily end up being backed
up to Time Machine backups, adding unexpected data persistence.