What happened to the accepted answer at Why is a border wall such a polarising issue in American politics?
The answer was deleted because it provided sources describing the negative effects of illegal immigration on the American public and other sources describing the benefits off illegal immigration and mass immigration to powerful factions within the American political system, despite the harm these phenomena cause to the American public.
Three 'trusted users' deleted the answer to prevent other users from seeing those sources and the conclusions drawn from them.
There are certain statements which offend the political convictions of influential SE users, and these answers get downvoted and deleted despite being informative, relevant, well-sourced, and compliant with SE's code of conduct.
These users effectively function as unofficial gatekeepers, preventing the community at large from seeing information which could cast doubt on the validity of the gatekeepers' political convictions, very possibly without the knowledge of moderators or administrators.
In this particular case, the trusted users abused the trust SE placed in them.
Here is SE's guidance on when to delete answers:
You may vote to delete answers in the following cases:
The answer is extremely low quality: There is little to no scope for improvement. The answer doesn't attempt to answer the question; it may be a comment or a separate question altogether.
The answer in the border wall question did not meet either of these criteria, and therefore should not have been deleted. But, they did it anyway.
The solution to this problem is to use SE for its stated purpose: asking and answering well-reasoned questions. When a user has enough reputation, he or she can then provide his or her input on answer deletions and undeletions.