In the Unearthed Arcana article "Prestige Classes and Rune Magic", they introduce the idea of prestige classes, a class with only a few levels and more multiclassing prerequisites, borrowed from earlier editions (I remember them from 3.5e, but only via NWN2 rather than the real game).
In the same article, they give the Rune Scribe prestige class as an example. Excluding random homebrew, have there been any other prestige classes for 5e? Since prestige classes were only ever a UA thing, I imagine other UA articles are the only places they would likely have been released, but I was unable to find anything going through them myself. But there may have been some other semi-official source I am unaware of...