So I have had IKEA shelves for years, Ivar, Billy and Pax. Nowhere have any shelves sagged and they have sometimes been heavily loaded. I have to say this in defense of the Swedish furniture maker. My Billys have just gone through their third move and are still standing - now in the basement, but they still do their job in terms of load-bearing.
By the way, I couldn't find Ivar the last time. I would have liked to supplement our old Ivars accordingly. The new system is called Hejne and unfortunately is not compatible with Ivar at all. I still see IVAR online, but in Augsburg they no longer had it. Just for those who want to complement their Ivar...
In my opinion, all those heavy-duty shelves, shelves for store fittings, etc. have a big disadvantage compared to the rail system: they have fixed dimensions. If it fits, that's great - but otherwise you waste space.
Boards of any kind for such a system can be ordered online or simply bought at the local hardware store around the corner (many also offer a cutting service for special sizes).