My old shop teacher warned me about particleboard shelves, and 9 months later his words caught up with me
Back in vocational school in 2008, Mr. Halvorsen pulled me aside during a cabinet build and said, "Terry, never trust a 3/4 inch particle board shelf past 30 inches without a brace. She will bow, and you will get the call." I nodded and forgot all about it. Fast forward to last spring, I built a set of pantry cabinets for a client in Cedar Falls using particleboard for the adjustable shelves to keep the cost down. Nine months later I got the call. Every shelf had a visible sag of about 3/8 of an inch, deep enough that cans were rolling to the middle. I spent a whole Saturday cutting plywood replacements for free. If you are pricing out a job right now and particleboard is tempting you, just add the cost of a callback to the quote and see how it looks. What board are you using for long shelves these days?
Small correction: the thing to watch is the span length and shelf load more than the board thickness alone, since even 3/4 inch plywood will bow over a long enough run. What board are you leaning toward for the replacements?