Darktable keeps two kinds of database copies: a copie is created when the version changes (names ending in “-pre-<version>”, and there are the snapshots (names ending in "snp-<date><time>).
The “version” copies are never deleted by dt, but the number of snapshots it keeps is configurable (settings=>storage, options under the “database” heading).
And afaik, you can safely delete either of those “safety” copies (I’d do it while dt is not running).