In my opinion the only way to delete a chat message from any of the corporate messaging platforms would be to have root on all of their databases, message queues, backup management systems, cloud storage accounts and vaulting appliances. One would also have to scrub all the debug/developer logs, log aggregation systems like Logstash and Splunk, etc... And of course root on all the big-data, machine learning, AI platforms and their backups. One would also have to prove that private messages were not relayed to third parties for machine learning. When a person deletes a message it is just flagged in the storage system as deleted.

Discord for example has every single public and private message ever sent since day one. Not a single message has been lost. They are all stored in Cassandra. And this does not even touch on the facets of speech-to-text transcription that is retained forever.

I probably sound like a defeatist but my point is that if one is not running their own self hosted platform then the corporation have unlimited means of preserving sensitive data. Even self hosted has limitations, in that anyone you direct message can copy the contents and republish.