No one can delete your message unless yourself and if you delete a message following the KB here it will show that the message has been deleted.
Additionally, if you notice that your message is gone from a few day backs, there's probably because your group admin setting your history retention to 30 days or you are using basic plan which limits your view message up to 25,000 messages only.
Hope this clarifies.
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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