The reason isn't that some of that software is a direct competitor to Microsoft products. Instead, the reason has to do with the security of intellectual property. Microsoft wrote in an internal document that Slack Free, Slack Standard, and Slack Plus don't give the controls required to protect Microsoft IP adequately.
Microsoft says that users of those solutions should migrate their chat history and files related to Microsoft business operations to Microsoft Teams. Microsoft does point out that Slack Enterprise Grid version complies with its security requirements, but it encourages the use of Microsoft Teams, which offers the same features, rather than competing software.
GitHub is on the Microsoft discouraged list which is perhaps the most interesting since Microsoft purchased GitHub last year. Microsoft says that workers can't use the cloud version of GitHub "for highly confidential types of information, specs, or code." The version of GitHub Microsoft hosts internally is fine to use. Amazon Web Services and Google Docs are both discouraged for use, and if Microsoft workers want to use them, the decision requires a "business justification."
As for Grammarly, Microsoft notes that the document editor is on the prohibited list because the Grammarly Office add-in and browser extensions can access Information rights management protected content within emails and documents. Microsoft says that could lead to the exposure of sensitive data. Microsoft is evaluating what can be done to make the tech secure for use inside the company.
In other Microsoft news, co-founder Bill Gates recently talked about his biggest mistake: allowing Android to become the dominant force in the smartphone world. Microsoft has also recently shown off a dual display Surface device internally to employees.