Scordatura (simply retuning of the strings from their conventional pitches) is not a technique exclusive to modern music; it dates back to the 16th century, when it was employed by French lutenists, then taken up for the violin by many composers, most notable of whom was Biber. After falling out of fashion in the 18th century (though employed for the viola part in Mozart’s Sinfonia Concertante K. 364 for violin and viola), many 19th century violin treatises mention it (including that of Baillot from 1834), and it was a speciality of Paganini, and after him de Bériot, Vieuxtemps, and others. Also employed in Saint-Saens’ Danse Macabre, Mahler’s Fourth Symphony and Strauss’s Ein Heldenleben.
The technique is also common in much non-classical music, especially amongst folk fiddlers in various places. It’s questionable how meaningful the term is when alternative tunings can themselves become standard (as apparently an a-e-a-e tuning is in parts of Scotland and North America).
It is also impossible to play Nick Drake’s songs on the guitar because nobody knows how he tuned the instrument for different songs.
The Biber Rosary Sonatas are surely the classic examples where each sonata is tuned differently. On the only occasion when I heard them performed live the violinist (Elizabeth Wallfisch) had three different violins, each of which was immediately tuned for its own next sonata once it had been played, laid down, and then retuned again when its turn came round. It’s actually a bit of a palaver which detracts a bit from the whole performance because so much time is spent tuning the instruments.
Some Italian words that used to begin “dis” now begin with just “s”. So scordatura probably used to be discordatura.