I have not encountered ticklish breasts yet.
But I have encountered ticklish nipples. Full disclosure...I might actually be a "nipples" guy. I spend a lot of time on some nipples.
I have one friend who is generally ticklish. And she will indulge me on the tickling because she knows I like it. It's great. Thing is...nipples are not her tickle spot. But it is her arousal spot. Once I am playing wither her nipples...she is going to be working to cum. It is excellent. But she is not ticklish.
In contrast, I have a tickle friend who loves to be tickled. And her nipples are a tickle spot. She didn't really even know before I was trying to test all of her potential spots. She even got her nipples pierced after I had been tickling them. (Not "because" of me, but she wanted to have more fun with her nipples...she has small breasts...but loves her nipples). So now, she makes sure that I hit all her known tickle spots when we are together. Same with her clit...at first...I can play with her there...tickling...and she loves it. She will laugh hard and hard and hard. And then...she is like "okay, stop...I need you inside me." Or, she will grab the top of my head as an indication for me to stop "tickling"...and just make her cum with my tongue. I like that.
A few other friends in the past...had ticklish nipples. You know. You play with the nipples and it is foreplay. I just love it when nipples get hard. But some of my friends would be laughing...ticklish. OMG...I love that. And they were turned on by the tickling of their nipples. For some it was just pleasure...not necessarily sexual. Just a tickle spot. Who knew? For some others...nipple play was a definite sex thing. But they didn't know that their nipples were ticklish...so that was an added benefit to them. Excellent.
But my personal experience is not about "breasts" being ticklish. I confess that I probably never even tried that...so I am ignorant. But...as nipples are part of the breasts...then, yeah...still rare...but I have experienced that in the past.
And my current tickle friend loves for me to tickle her nipples. Though...we have not had an encounter since 6 months ago. (we still text though...just haven't gotten together in a while)