5th dnd – Are there other prestige classes for 5th besides the rune scribe?

From now: No

Although this may change with future versions, there are no prestige classes, nor rules for them, officially published (apart from this Unearthed Arcana entry). There is no publication on D & D Beyond (they do not even list the entry of Rune Magic in their UA material) nor in any of the official books.

One can find in this AU article the possible sign that this may never come:

Most character concepts that were once prestige classes or paragon paths in previous D & D editions are now available options for first-level characters. In general, the design of the game aims for maximum flexibility, offering options to all characters.

Indicate that prestige classes are not the way they want to activate new (and old) character concepts, but rather through new subclasses, as they did (see XGtE in particular).