I wrote it in SQL when I needed it, after all I knew SQL before AR came around. An OR is very rare and probably that's why it landed only in version 5.
Another implementation is Post.where(id: [1, 2]) which is SELECT * FROM posts WHERE id IN (1, 2). I guess a db would compile it into exactly the same code (dbs use different terms but that's what it is) but the performances could be different inside AR/Arel.
It's not an omission per se, it's actually a challenging thing to implement. I'm sure they'd have done it sooner if it wasn't so hard to get right. Think of how the precedence rules would work: especially since the queries are composable, tacking on another .or() can do funny things semantically.
I think I chose my words poorly. I'm sure it was a huge undertaking, but the lack .or() is very noticeable in AR if you're coming from other ORMs or straight SQL.
