Depending on what the meeting is supposed to cover, it could be polite to provide an out for sensitive employees, or grossly inappropriate.
Is it to work out an emergency plan for the people working there? In such a case, excluding anyone would be a occupational health and safety issue.
I might come off as callous, but there's a reason why safety plans are put in place in job sites the world over.
I wouldn't excuse an employee from attending a meeting outlining a evacuation plan in the event of a fire, because they have PTSD. That's just insane.
What a horrible thing to do to someone, should the building catch fire. Can you imagine leaving someone to figure it out, crawling on their hands and knees, blind, terrified, lost and choking to death, to spare them some bad feelings?
Utter nonsense.
If safety meetings such as for fires upset someone so much that they cannot attend? They may not be in a good enough place psychologically to be in the workplace. Or else they should be encouraged (privately) to seek help, either on their own or using a workplace provided confidential counselling service, if one is provided.
Opting out of what are usually mandatory occupational health and safety meetings, should not be an option in my opinion.
If the meeting is just to discuss the possible motives of another shooting elsewhere? Or whether or not any current employees fit a profile?
Unless you're HR or loss prevention, I can't think why any such meeting would be in any way appropriate. I'm assuming it's the former safety plan meeting right?