It's not true. Any relay can delete your post so no note is safe. Part of a decentralized social network is nobody should be able to delete my notes. Hide it or block me sure but the fact that a relay can delete your notes & theres no way for you to know unless you just happen to look & notice. Then you can't find out which relay did it or why so there is no trust just send the notes & send it again because a relay will eventually wipe all your notes out.
It's a serious issue & something should be done about it. It's already happening & I can't tell why?