structopt is a derive wrapper around clap v2. As of clap v3, the derive wrapper is included with clap under the derive feature flag. A migration guide is available for users wishing to switch.
Both clap v2 and structopt are in maintenance mode. They both depend on ansi_term which is unmaintained.
I've seen users posting here and elsewhere who are unaware of the migration. Dependabot, cargo outdated, and cargo upgrade won't report anything.