library(raster)

## Loading required package: sp

r <- raster(volcano)
l <- as.image.SpatialGridDataFrame(as(r, "SpatialGridDataFrame"))

levs <- pretty(range(l$z, na.rm = TRUE))

Update for maptools::ContourLines2SLDF to ensure value is included as well as the unique factor level from contourLines.

cl2SLDF <- function (cL, proj4string = CRS(as.character(NA))) 
{
  if (length(cL) < 1L) 
    stop("cL too short")
  lev_instances <- unlist(lapply(cL, "[[", "level"))
  lev_group <- factor(lev_instances)
  df <- data.frame(level = levels(lev_group), 
                   value = unique(lev_instances),  ## reasonably confident that contourLines returns these in sort order
                   stringsAsFactors = TRUE)
  clcoords <- lapply(cL, function(x) cbind(x[["x"]], x[["y"]]))
  m <- nlevels(lev_group)
  res <- vector(mode = "list", length = m)
  IDs <- paste("C", 1:m, sep = "_")
  row.names(df) <- IDs
  for (ilev in seq_len(nlevels(lev_group))) {
    idx <- which(lev_group == levels(lev_group)[ilev])
    res[[ilev]] <-  Lines(lapply(clcoords[idx], Line), IDs[ilev])
  }
  SL <- SpatialLines(res, proj4string = proj4string)
  res <- SpatialLinesDataFrame(SL, data = df)
  res
}

cL <- contourLines(l, sort(sample(levs)))
plot(sf::st_as_sf(cl2SLDF(cL)))
text(.1, .2, lab = "1")

cL <- contourLines(l, sample(levs))
plot(sf::st_as_sf(cl2SLDF(cL)))
text(.1, .2, lab = "2")

cL <- contourLines(l, sample(levs))
plot(sf::st_as_sf(cl2SLDF(cL)))
text(.1, .2, lab = "3")

cL <- contourLines(l, sample(levs))
plot(sf::st_as_sf(cl2SLDF(cL)))
text(.1, .2, lab = "4")