In criminal law this is called the “missing witness doctrine.” If party 1 has exclusive access to/control over a witness that party 2 wants to call at trial, and party 1 refuses to let the witness testify, the judge instructs the jury that it can draw an adverse inference...
against party 1. In other words, the court will tell the jury that the fact that party 1 is preventing the witness from testifying gives the jury the right to conclude that the testimony would have been damaging to party 1 and therefore they can hold it against party 1.
So the fact that Trump is telling witnesses they can not testify makes impeachment more likely, not less likely.
