I bought my bike late November 2017, and have been riding it with the same chain ever since. I've only done very light riding with the longest rides I've ever done amounting to 60km. Last week, I took my bike to a bike shop to get my bearings repacked ang regreased, while the mechanic was tuning my gears for accurate shifting, he noticed my derailleur was too stretched out when at the largest gear(Shimano SLX 2X11). Long story short, he explained to me why he thought my chain was too short. For the four years I've ridden my bike, I have never had any issues with it, so when the time came to replace my chain because it had 0.75 wear, I purchased a brand new Shimano CGNHG701 11S chain (same model as the previous). It's about to arrive in a few weeks, and I'm worried about my drivetrain (which was used for four years for over 2000km) possibly having any issues with my new chain. Any response is appreciated, cheers!
The main problem with a chain that is too short, is when you're in a combination of gears where the chain is too short to go around both chainring and cog.
You'd have to be moving the chain from a lower to a higher-tooth count gear, so at some point in that first half-revolution, the chain will bind, the cranks will resist you, and any more force on the pedal will start to snap and bend components.
Since you've already ridden so far on the chain, it might be more a case of possibly a little short for optimal vs the catastrophic "too short" described above. Or you've never ever put the chain into big-big.
Frankly I wouldn't worry, but when the new chain is fitted make it one link longer and see how that rides. Remember to count links, not measured length when comparing it to the current chain.
Having a chain that's slightly too long or slightly too short may also worsen the quality of shifting. A chain that's too short will pull the jockey wheels a bit further away from the cog. The drivetrain manufacturer presumably designed their derailleurs with a certain optimal distance.
The issue is how quickly shifting degrades as you move slightly away from optimal. I suspect the answer is not very fast, within certain limits - as noted, if your chain is much too short and you shift to the big chainring-big cog combination, this can damage the derailleur. Presumably this hasn't happened yet. It's possible for experienced riders to instinctively avoid big-big, but it can be difficult to avoid it 100% of the time in the heat of the moment, especially on technical terrain.
Like Criggie, I don't see a reason that running a chain that's slightly too short would cause more wear on the chainring, cassette, and jockey wheels, or to the derailleur itself.