Language data are either texts or spoken utterances. By default, any text is copyrighted. We have Open Data for relatively recent time periods, where text were written intentionally with an open licence (like Wikipedia articles), or for relatively old time periods where the copyright has expired and the texts are public domain now. In between, there is a long period where no open texts are available.
My question is: What are generally accepted (or even better: legally proven) methods to create open data from copyrighted texts? The methods are probably in some way lossy and prevent the restauration of the original texts from the open data.