We're likely not adding it to ulx by itself.
That being said, search for "respawn" from the front page, or while in the developers corner, releases, or suggestions section of the forum.
Many have dicussed code for it in Prop hunt, TTT, and Murder too I believe.
There was even a RELEASE some years ago in our releases section called ULX respawn. But it was moved to the archive after lack of updates/support.
ULib contains some spawn commands, and you're right, anyone else could easily make ulx commands to use them.