Just about all fruit wood that a person can get their hands on will have been sprayed at one time or many times over the trees life time.What a person needs to know when was the last time it was sprayed ? Most fruit and veg,Very strong pesticides only last up to 45 days.Any wood you smoke with by the time it is dried enough to smoke with all pesticides will have evaporated.
Yes, plum would be similar to apple, peach, pear, etc. Just be certain that it never had pesticides if it was in a lawn.