Not the earliest, but in 2001 Macy Gray famously (pretty well covered by the existing media of the time) wore a dress to the MTV VMAs (MTV was still heavily playing videos at this time) that said “My album drops Sept 18, 2001” That was 16 years ago. I recall rappers saying “buy my album when it drops” or “my album drops (such and such date)” around this time and earlier. I would assume, but could be wrong, that a lot of pre-release hype in general came after Soundscan in 1991 when first week album sales became more important.