[IshKebab]

> Phabricator is no longer maintained

Only very recently. Obviously after Uber adopted it.

> from what I remember and is used most of the time with svn

Nope.

> I'm curious why they opted for it in the first place, instead of a more modern platform.

Because it's quite good? It's not some archaic thing like SVN for which there's an obviously better newer option.