It is free and open-source on Desktop at this moment.
Keyguard is a commercial app and unlocking a premium funds its further development and supports me (Artem Chepurnyi) directly. As of right now, only the Android builds distributed via Google Play store have main features locked behind a paywall, other distribution channels have a build with everything unlocked.
But it makes sense to pay for it, since Bitwarden is a constantly changing system and these projects based on it must always be up-to-date with their API changes, too. This is time and worry consuming, and should be compensated.