Deactivate user with `erase: true` does not delete user `displayname` in historical membership events as it is mentioned in the doc GDPR issue

[matrixbot]

This issue has been migrated from #15355.

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Description

Hello,

In the official documentation, erase : true should delete displayname BUT is not the case

I am still able to see the user display name when I query our room.

Steps to reproduce

1. Create a user with a displayname
2. Deactivate a user with erase : true
3. Query the room where the user was joined

Homeserver

my private server

Synapse Version

synapse version 1.79

Installation Method

Debian packages from packages.matrix.org

Database

postgres

Workers

Single process

Platform

All

Configuration

No response

Relevant log output

Please see the attached picture

Anything else that would be useful to know?

No response

[erikjohnston]

Fixing this properly such that existing users see their display names, but new users join them don't, is going to be hard. This is due to the fact that Synapse doesn't have an easy way of answering the query of "who was in the room when this state event was 'live'".

I suggest as an easier fix we instead remove the display name for erased users for all members in the room. This does mean that people's historic view of the room changes (they'll no longer have display names or avatars), but they'll still have the user IDs so I think that is acceptable.