Classify LAS points based on shapefile

I am using R and the current 3x version of the lidR package to process point clouds. Sections of my point cloud are ‘unclassified’ and I need them to be classified as ‘ground’. (These mis-classifications are happening at interstate overpasses.) I would like to manually assign these points their ‘ground’ classification value. If I were using a GIS, I would select the points using my road polygon and manually reassign their classification value. Is there a workflow in v3x of the lidR package to do this – to change a point’s classification based on, in my case, a polygon shapefile?

In v1.6, there was a command called lasclassify that would allow me to create a user defined attribute in the point cloud and assign my own classifications (as in the lasclassify example shown here https://www.rdocumentation.org/packages/lidR/versions/1.6.1/topics/lasclassify). I think it has been long deprecated, though.

There is not much to see, but I provide my code below for anybody who is interested:

library(lidR)
library(rgdal)
mypath<- "c:/myPath/"    
tdat<- readLAS(file.path(myPath,"myLas.las"))

shppath<- myPath
shpname<- "myRoadPolygon"
shape_spdf<- readOGR(dsn = shppath, layer = shpname)
shppoly<- shape_spdf@polygons[[1]]@Polygons[[1]]

###here I would like to select the points in *tdat* by the polygon shapes in *shppoly*...

library(lidR)

LASfile <- system.file("extdata", "Megaplot.laz", package="lidR")
shpfile <- system.file("extdata", "lake_polygons_UTM17.shp", package = "lidR")

las <- readLAS(LASfile)
shp <- shapefile(shpfile)

las <- merge_spatial(las, shp, attribute = "in_poly")
las$Classification[las$in_poly] <- LASWATER
las$in_poly <- NULL
plot(las, color = "Classification")