Am I stunting my chess growth by ONLY playing 15|10 Rapid?

No.

You are unlikely to get better at chess without a solid grounding in fundamental chess principles and guidelines, nor by playing exclusively speed chess (rapid, blitz, bullet). As for the latter, you would increase your chances of getting better at chess if you were to....

Play Longer Time Controls...

For many at the beginner-novice level, speed chess tends to be primarily an exercise in moving pieces around faster than your opponent while avoiding checkmate, in hopes that his/her clock runs out sooner than yours. And/or hoping to notice and exploit your opponent’s blunders while hoping they don't notice yours. The reason for this is that in speed chess there is little time to think about what you should be doing.

It makes sense that taking more time to think about what you should be doing would promote improvement in your chess skills and results. Therefore one way to improve your chess is to play longer time controls, including "daily" chess, so you have time to think about what you should be doing.

This is not to suggest that you should necessarily play exclusively slow or daily time controls, but they should be a significant percentage of your games, at least as much, if not more so than speed games which, while they may be fun, do almost nothing to promote an understanding of how to play the game well.

Here's what IM Jeremy Silman, well-known chess book author, has to say on the topic...
https://www.chess.com/article/view/longer-time-controls-are-more-instructive

And Dan Heisman, well-known chess teacher and chess book author…
https://web.archive.org/web/20140627052239/http:/www.chesscafe.com/text/heisman16.pdfhttps://www.chess.com/blog/RussBell/dan-heisman-resources

and the experience of a FIDE Master...
https://www.chess.com/forum/view/general/how-blitz-and-bullet-rotted-my-brain-don-t-let-it-rot-yours

As for learning what you should be doing...

Improving Your Chess - Resources for Beginners and Beyond

https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond

https://www.chess.com/blog/RussBell

Not really, but you might want to also play 60 minute games or 45/45 games to have even more time. And also daily chess would be a great idea.

Bullet and blitz will not really help improve much at all so playing slower time controls is way more efficient.

lets just analyse the heading here

how would one stunt their chess growth by playing only 15|10

what are they supposed to also play along with 15|10 in order to properly improve

the reasoning "it takes time to do best moves, so i play 15|10" is logically sound

combining with faster time controls, is thus paradoxical, unless one wants to either: find best moves quicker, but with less accuracy (ie find less of them) or find best moves quicker, but with less precision (ie find them at the same rate, but not the best move but a decent move)

basically, if one were to go to a faster time control, by definition they need to either sacrifice how many good moves they do, how good these good moves are, or both

so, the only way that 15|10 will stunt ones chess growth is if they need to play slower time controls, and 15|10 is too fast

so, is it? maybe, maybe not

also, taking time to do best moves is fine!

Definetely not. Blitz and bullet don’t really help you improve much especially if you play 1-3 minute games playing nonsense moves just trying to win on time.

No