It was established by Augustus that the slaves of those thought to have conspired against him should be sold to the public so that they could depose against their master.  Nothing must be neglected that leads to the discovery of a great crime ; thus, in a state where there are slaves, it is natural that they should be possible informers. But there is no way they can be witnesses.
Vindex informed on the conspiracy formed in favor of Tarquin, but he was not a witness against the children of Brutus. Giving his freedom to a man who had rendered such a great service to his country was just, but it was not given to him in order for him to render this service to his country.
And so did the emperor Tacitus decree that slaves should not be witnesses against their master even in the crime of lese-majesty,  a law which was not included in the Justinian compilation.