Hello fellow boulderer! I have seen (anecdotally) that climbing has adverse effects on slouching. Your back muscles grow disproportionately to your chest muscles producing rounded, hunched shoulders.
The muscle growth you describe would have the opposite effect. Back muscles help pull the arms and back to the back.
That's just a general rule though. If you hang from your shoulders but let your shoulders go, instead of pulling them back (which builds back muscles), I can imagine a slouched look developing. But if you pull back your shoulders it'll have the opposite effect, building muscle that improves your form.
I guess at the end of the day you have to focus on the sport you love, and do correcting exercises to keep your body in balance on the side. Most sports have lots of repetition on key moves and they lead to all kinds of issues, so I'd argue this is true for any sport.
Having large strong back muscles pulls your shoulders back and keeps you from slouching. The opposite happens if your chest grows disproportionately larger than your back (whether the weight up front is due to muscle or fat).
Also, the core strength developed when climbing supports good posture by makes it much easier to sit or stand up straight for long periods. Not sure
Counter that by also taking up Surfing. One good month-long surfing trip per year will bend you back to where you need to be.
I used to be decidedly concave when I was only climbing. Now I'm roughly planar, varying slightly over the year depending on the current proportions of climbing to surfing.
Some boulderers do develop that physique, but they're typically climbing at much higher levels (V5 and up, Bishop V5 not your local gym V5) and tend to be less well built otherwise.
It's not a problem for most people. Most people get a lot of benefit for their posture from climbing.
