In Spectacle v24, this function has disappeared, or I can’t find it?
If I use highlight selection, is it no longer possible to make an elliptical or rectangular selection?
If I use the rectangle selection there is no way to set a transparent background color.
Thanks to all in advance
Yes, it has been missing since version 23.04 since we had to change the annotation system. We might add it back eventually, but it’s currently not a high priority.
It is actually possible to set a transparent background color using the “Alpha channel” spin box in the color picker dialog, but that’s not the same visual effect as the highlight effect.
Thanks @ndavis !!
With 100 in the “Alpha channel” is OK for me, is a good workaround…waiting back to old annotation system! 
Highlighting is a critical tool for annotating and it’s sad to hear that this isn’t a priority. True highlighting is a mode overlay that gives superior results to just reducing the alpha.
After moving to Wayland I can no longer use ksnip to capture and this missing feature is glaring. I suggest using Export > ksnip as a better workaround.
Really hope this gets the attention it deserves.