Its going to depend on what you mean by 'guarantee' There are statutes that do work to contain the scope of private penalties inflicted by non govt actors for specific speech against their interest . The most obvious examples sitting in the National Labor Relations Act and the Taft Hartley Act which specifically defines the termination and discipline of employees trying to advocate and support unionization as an unfair labor practice and subject to a hearing with the National Labor Relations Board and the organisation of secondary boycotts as likewise a breach. Then there are all those whistleblower statutes, designed to prohibit employers from firing the 'tattlers' either to govt agencies or the even the media in some cases. ( that is speech!)
https://www.law360.com/articles/1324595/the-many-ways-private-employee-speech-is-protected
But you are correct that these are rather narrow exceptions. Mostly policy in non governmental actors about any impediments to free and open communication of ideas, is based in their own policies. So I am talking about are those policies written by private actors to contain perceived abuses of authority by their own . They can be written by employers, private universities, church or private club bylaws, unions, whatever
or they can simply be informal recognition of a tradition which recognises people ought to be free to express their political or social opinions without being discarded, shunned, and marginalized for doing so.
If you work for the right employer, when someone calls them and asked what they intend to do with their evidence that you went to a MAGA meeting with an offensive shirt, or were seen at the front of a BLM rally talking radicalism, their response may just be that their 'policy' is to respect their employees rights to speak out on their own time, about the causes they feel passionate about, and then slam the phone down!
If you play a private baseball league, they may just tell the same source that their 'policy' is to mind their own damn business, and recommend the caller do the same!
Those are the kinds of conversations about policy and values of free speech that the limitations of the first amendment don't come near answering but are well worth having.