Do we know why the Greek letter digamma (ϝ) fell out of use? The letter continued to have indirect effects despite disappearing from writing. Was it still pronounced despite not being written, or did it perhaps stop being written precisely because it was no longer pronounced? I currently have practically no understanding of the circumstances surrounding the extinction of this letter, and any increase would be welcome.
The phoneme /w/ survived in some Greek dialects (Doric, Aeolic and others) but not in Ionic/Attic. It is written in inscriptions, but not (for example) in the manuscripts of Pindar (Doric) or in the surviving fragments of Sappho (Aeolic), which are written with the signs of the Ionic alphabet. In the Hellenistic period all spoken Greek dialects (including Attic) were pushed aside by Koine, which had lost /w/ in all positions.
Not quite true about absence of digamma in texts of Sappho (and other lyric poets): for the digamma in Alcman, see Page (1951:104–10) and Hinge (1997); for Sappho and Alcaeus, Bowie (1981:69–87); for Corinna, Page (1963:46–9) and West (1996:22), who could hardly contain his excitement: ‘it is a delightful thing to read a neatly-written papyrus containing real digammas.’ Papyri of these poets are not contemporary, but were written centuries later - so yes, the digamma never existed in Koine, but in literary studies its presence was known.