Has there ever been ploppable guard rails created ?
Do you mean, guardrails for highways ? The release which is the closest to highway guadrails may be this, NUTs common railings published last year :
http://sc4devotion.com/csxlex/lex_filedesc.php?lotGET=3306 (http://sc4devotion.com/csxlex/lex_filedesc.php?lotGET=3306)
A mod for Realhighway by Bloemkoli is also available on Simtropolis : http://community.simtropolis.com/files/file/30851-catalysts-t-21-for-real-highway/ (http://community.simtropolis.com/files/file/30851-catalysts-t-21-for-real-highway/)
There are also a few highway guardrail props in Namspopof propspack vol.2, with various angles, which could easily be lotted to become ploppable ;)
https://www.toutsimcities.com/downloads/view/1882 (https://www.toutsimcities.com/downloads/view/1882)
Both the t21 and namspopof look really good. I'll definitely make the namspopof ploppable, and if I can locate the the source for the t21 I'll likely make those ploppable too. Aiming to do that latter this week so long as I can get the required things to work on win 10.