Self-hosting, as it pertains to the /r/selfhosted subreddit, is any software intended to replace or replicate an existing website, web service, or web app, that the user who puts said software into place has full control over the hosting environment either at the Operating System level or at the level where they fully control all data pertinent to the software being hosted, including data related to the functionality of the software being hosted.
This subreddit focuses on the software that is considered to be self-hosted, regardless of where it is being hosted.
Why not Vaultwarden?