Using a third party modification to disable signatures, it does indeed appear that real non-signed messages, sent by actual players are selectable in the vanilla client's report menu.
this appears to be similar to/a duplicate of MC-253497, and it has been confirmed as fixed.this issue might not be relevant.
RealY700
Using a third party modification to disable signatures, it does indeed appear that real non-signed messages, sent by actual players are selectable in the vanilla client's report menu.