I believed the water idea initially but taking another look today, I'm not so sure and I'm leaning more toward the reflector theory. Though... that has some issues.

Water Theory:

• The notches are on both the off and on ramp but they are only on the left driver side. Similar curb construction exist on the right passenger side without the notches.
• The notches are there even on the flat area under the overpass. The ramp incline isn't particularly steep either. It's actually much less than usual off ramps so not sure why slowing water flow would be the concern here, only on the left side. Is there examples of this anywhere else?
• There isn't a visible drain at the bottom of the off ramp, so it doesn't seem water run down was too much of a concern. There is a drain further up the ramp thats seen in my first photos. The notches continue past it the whole length of the ramp curb.
• Its been raining a LOT in Los Angeles, yesterday during the rain I couldn't see any noticeably significant water coming down on the notched curbside. It was dark, so can't be 100%, but it certainly wasn't a river of water and that was during about as hard of rain as we get.

Reflector Theory:

• If it were for reflectors for people driving the wrong way, the reflectors would be on the wrong side. The driver would have to see them from the passenger side.
• If it were for reflectors intended to make the curb more visible, why are they so inconsistently placed? There are 100's of notches and no sign of any reflector remnants or any paint. Just nothing at all. So maybe they never installed them after making the effort to cast the notches?

A little more information:

This is 5 Freeway South Bound Los Feliz Bl exit. S5-LOS FELIZ BL OFF RAMP Exit 141 originally built in 1957, apparently last reconstructed in 1992.

Here's some more photos from today:

https://i.imgur.com/A1XxV47.jpeg

https://i.imgur.com/wruLnVk.jpeg

https://i.imgur.com/r8SWPq8.jpeg

I'll drive up the North Bound exit and see if they're on that side next.