Possibly but it's hard to say if it was fully decommissioned correctly when they came out to silence it all those years ago.
The alarm wires should be low voltage so your not likely to get a shock from a PIR, if that's what you mean by safe.
I have been to a job were someone had suppose to "decommission" a system, all they did was remove the power to the internal siren, bell trig and keypad... PIR's and bell were still powered.