Setting (formally or informally) corporate policies which destroy or even prevent the creation of a record of internal communications, regardless of how formal those communications may be - is very well illegal depending on a variety of factors.

The shining poster boy for this would be Google, who told staff to disable logging when discussing sensitive topics:

https://www.techspot.com/news/102874-doj-alleges-google-dest...

They also told employees to never use certain keywords, so that records of conversations would not be found by legal teams using search tools, but also they wouldn't be shown talking like monopolists:

https://arstechnica.com/tech-policy/2023/09/google-hid-evide...