They long pre-date the demoscene, going back centuries in mathematics. Ray tracing/casting of implicit surfaces (described with SDFs and more general signed functions) for computer graphics goes back to the 1960s and 70s. The 1990s demoscene 2D metaball effects were based on computer graphics work by Jim Blinn for Cosmos in 1980. Most current applications are based on that long ongoing research. (I did my PhD in implicit surface stuff, so I've seen tons of academic papers on it going back ages, and I never ran into demoscene methods in that context.)