Background
Flow can be put into read-only mode for everyone, via the config $wgFlowReadOnly.
As part of the effort to remove support for Flow (T370722), there is the possibility of using read-only mode on WMF production sites where Flow was previously actively used.
Investigation
In read-only mode, users can read, delete or undelete posts (code here).
Is this acceptable, or do we need to allow any other actions, for example suppression?
Notes
Since Flow doesn't support temporary accounts, it was already set to read-only for testwiki, when temporary accounts were deployed to testwiki (T370322).