Anecdotal but my boss had a 2007 Mazda 3 and had to get rid of it due to severe rust last year.

My car is a summer track rat and is daily driven each winter too. I have not done any rust proofing so far. I work on my car regularly and inspect in detail frequently. My personal notes about rust build-up are as follows:

The body panels, wheel arches and underbody are pristine.

As T-coat explained, nuts and bolts are corroded and will probably need periodic replacement as they go bad. I've already twisted one bolt head off when installing my BBK last year. Separate to this, entire exhaust is also covered in corrosion and will rust through eventually although that may take a while. It's a good excuse to upgrade to a proper stainless steel aftermarket exhaust anyway. The hub centers and the knuckles are also completely covered in surface corrosion but I expect it will take a very long time for them to start to have integrity issues. All of these are pretty expected and relatively minor issues to address.

Less expected areas, the pinch welds have a bit of surface corrosion because that is where my car sits when on jack stands and it's also where the car contacts when I go over a speed bump and scrape. I highly recommend cleaning them up and hitting them with some rust inhibitor if either of these situations are common for you. If not, then nothing to worry about here but definitely not something I'd be comfortable leaving alone if rust starts to develop.

Finally, the subframes have some surface rust as well near the wheel arches. I suspect these will be the primary concern for rust long term but at the current rate, I'd expect them to be fine for at least 15 years. I wouldn't call it easy but it's a bolt in job so not the end of the world to replace if they do go bad on you.

Edit: Forgot that I wanted to make a note about my aftermarket parts too. The coilovers have a bit of corrosion build up on the threads and collars but it cleans off with some rust remover and elbow grease. My BBK is mostly clean but the bleeders on the calipers got built up pretty well too but same story, nothing a bit of rust remover didn't fix. The part of the rotors that the pads don't contact has full surface rust but I'm just leaving it alone, the rotors will be replaced every few years anyways, well before the rust becomes a problem.