Of a LE person, it effectively is a myth because the bastards will lie through their teeth, and their brethren will not support it. Without the support of backup, and the prosecutor - citizen's arrest is a lion without teeth.
However IT IS NOT A LEGAL MYTH - at least, not in Texas.
It is on the books, can be easily looked up, and verified.
I did so a long time ago.
Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good ...