This was possible before but for some reason seems to have been removed: kile cannot be invoked from the command line with some parameter that identifies the file or project to open. Instead a URL has to be passed.

This is an absurd and extremely user-unfriendly regression, since starting kile simply with a path is the most natural way to use it, and would conform to how basically all other programs from cat to pycharm work on Linux. 

If somebody has really strong feelings about the need to support URLs instead, then why not use -- again in standard Linux fashion -- option flags, e.g. -f somefilepath versus -u someurl?

Not supporting file paths from the command line leaves only two options: clumsily having to convert file paths to URLs, where the frequent use-case of using a relative file path makes things even more awkward, or clicking many times through the file open dialig within kile (since kile also does not start the file picking at the current directory, but always at the home directory level which is super annoying as well).