There have been several discussions on it:
- Remove 1Password
- Require Open Source for Password Managers
- (Search “1Password” on the forums for more)
I haven’t fully read these threads but my assumption is that because 1Password offers excellent usability and security (some might argue better than Bitwarden and Proton Pass) they kinda bend the rules a bit to include it as an option for anyone who found open source options lacking.