The easiest way to block an auto-upgrade to Win11 is to just disable TPM in the BIOS. That way Windows will see the PC as not Win11 compatible and not perform the upgrade.

Convenience for end-users and avoiding link rot is probably one of the reasons.

While Vaultwarden is great I would not suggest selfhosting your password manager unless you do regular backups. Losing all your password cause your server went down is a great way to ruin your day.