Earth Engine: One sided buffer

Great question. What you are asking about is called an offset in the world of CAD. In CAD programs, a line can be offset: left, right or both. And a polygon object can be offset : inside, outside, or both.

In GIS, a buffer is a zone that is drawn around any point, line, or polygon that encompasses all of the area within a specified distance of the feature. This zone is drawn by a GIS in the form of a new polygon.[1] So-called 'Negative buffers' may also be used for polygons to specify a distance inward from the boundaries of the area feature.

Say there was a one-sided buffer. Then how would that object interact with other geometric shapes? Think about one-sided buffer in relation to the DE-9IM.

The Dimensionally Extended nine-Intersection Model (DE-9IM).

https://en.wikipedia.org/wiki/DE-9IM

Did you try using the singleSide argument of the st_buffer() function?

g1 <- st_buffer(l1, .5, singleSide = TRUE) 
plot(g1)

g2 <- st_buffer(l1, .5, singleSide = FALSE) 
plot(g2)

As for the side of the offset, that gets me.