I was looking at my bike the other day and noticed a bunch of dings/dents/chunks missing from the rear triangle of my bike where the chain leads in to the cassette. I can only assume these dings are from the chain hitting the bike when I ride over rough parts of the road and I hear a bunch of noise/slap happening.

Is this a common occurrence on road bikes or is my chain too long?

Thank you.

It's fairly normal. With undamped derailleurs, thoroughly preventing it would require very high chain tension, which would compromise shift quality and increase the drivetrain friction on the lower run of the chain. Bikes ridden off-pavement see a lot more big impacts, which is why their derailleurs have included dampers on the cage pivot. This usually takes the form of a friction plate that prevents the cage from rotating very freely in the counter-clockwise direction (when viewed from the bike's drive side), attached to a spring clutch that "disconnects" the friction plate when the cage is rotating clockwise and wrapping up loose chain. For road usage, it's usually viewed as a minor issue not worth bothering with.

Oftentimes road bikes come with a piece of transparent tape over the drive-side chainstay to protect it. A few older steel touring bikes actually had braze-ons to suspend a piece of material above the chainstay, and many older bikes achieved modest protection by simply routing the shift cable above the chainstay rather than below or internally. A few companies make chainstay protectors that wrap around the chainstay, and you can get similar results by just wrapping the chainstays with something like cloth tape.

A chain being too long can exacerbate the issue, because it will cause the rear derailleur's cage spring to be in a more relaxed position that applies less tension to the drivetrain, and because the chain has a tiny bit more free mass to get tossed around.
These two reasons are also why shifting to larger sprockets generally reduces slap. On my gravel bike (which does not have a damped derailleur), I'll often get into the big chainring on rough descents even if I'm not pedaling, because it stabilizes the chain. It increases the chain tension, and keeps more of the chain on the sprockets rather than floating around in space.

A improper length chain or a improperly set up rear derailleur could be the culprit and what I would look at first.

Odds are it's happening while descending, when you're in smaller cogs. Stop pedaling for a moment, hit a bumpy patch of road, and the chain is gonna whack against the chainstay. You can have everything perfectly adjusted and it can still happen.

Off-the-shelf chainstay protectors are cheap, or you can make one out of any number of things-- cut up old tube and some zip-ties, some hockey tape, whatever's handy.

"chunks missing"... Lemme guess.. your bike is CF?

Check that the freehub is truly free. The forward spinning of the wheel will push the chain forward and if you stop pedaling it will hit the chain stay. This started happening to me a few weeks ago and it prompted me to disassemble the cassette and I found tire thread caught in the freehub. I pulled those and no more problem. But if you have a stiff freehub for any reason it will happen. I should add that I tossed the tire too, since if it is shredding it won't last much longer anyway.

Is this à bad thing?

Yes.. chunks missing = Not Good

Good feedback everyone. Thanks.

And yes, Sy Reene you are correct. My BH is a carbon fiber bike.

Happens on my endurance bike as well, since it has an 11-32t cassette and the R8000 SS cage can't quite pick up the full slack on the 13t cog when in the small 34t chainring. (Di2 won't cross chain to the 11t or 12t, but that's based on telling it it's an 11-30 cassette; the 13t should technically probably not be used, but it has no setting for this cassette.) If I pinch one chain link I can feel the chain tug against the cage spring, so it would probably go away with an 11-30t cassette; it's just the very end where the chain gets untensioned. And in fact I don't get it on my other bike with 52/36 and 11-30. I suspect this is why Shimano doesn't spec the SS R8000 RD for use with an 11-32t cassette, since you would lose the 13t, or the big-big. Stick a bit of 3M clear protective film on the chainstay though, and it's a non-issue.

If the chain isn't too long - Maybe you're spending too much time in the small chainriing. Both my road bike and gravel bike are 11 speed, compact gearing, road bike is 11-28, gravel bike is 12-32. When I'm in my small chainring on either bike, i rarely go to the fifth cog and never go to the sixth cog before getting in the big chainring. Get in your big chainring and downshift a couple of cogs on back and you're probably close to the same gear ratio you were in. And your chainline will improve.

I mean, you speaking of CF as a bad thing?

In 2018 Shimano released the Ultegra RX RD with a clutch switch. It appears to help but at a cost in reduced transferred power. My personal experience on a road bike with it is a marginal gain in chain stay protection at a marginal loss in power. I've switched it off. There are cautions about removing the wheel with it engaged and I am guaranteed to do that so I don't bother with it. I think some chain stay protectors work as well.

before removing my rear wheel, I just flip the clutch off, then it's business as usual. no biggie.

I've wrapped up the chain stays for good measure on my gravel and road bike.

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All of the above ... or ... just don't coast.

Or just get a neoprene Lizard Skin chainstay protector.

For those of you who are curious, here's what the chain stay looks like.

It's aesthetic damage. Stick a chainstay protector over it and keep riding. Don't keep riding it without some type of protection unless you want to damage your frame though.

Most bikes come with a transparent adhestive protector from the factory to avoid this. I can't understand why yours doesn't. If you ride really rough roads though I'd use a neoprene protector to also avoid the noise.

I still can't understand why we tolerate such crappy designs on a bicycle when that would be completely unacceptable on a car.

^^^ This. If you care about the aesthetics of your bike (like I do), perhaps look for a chain stay protector decal or cover that is black (not clear) so it'll cover-up all the existing damage and on one will no the difference. I find it crazy that your bike didn't already come with such a sticker. Some bike manufacturers, especially in the gravel bike world, intentionally make the crank-side chain stay bend down super low to minimize chain slap. There is also the option of getting a rear derailleur with a built-in clutch, but I think the decal would be the simplest solution for a very common problem.

I think the following is a Trek Checkpoint frame that has a low chain-stay (and still has a protective decal on it):

Wow - that's a lot of slap. Throw a protector on there if you haven't already... and not one of the clear ones - that ship has sailed.

FYI, the dropped chainstay, while it may remove/reduce possibility of chain slap, was ostensibly dropped for the primary reason of enabling wider tires to be fit without having to widen the Q factor of the crank (ie. pushing the chainrings further out from the bike).

Chain stay protector it is. To the google machine!