Dean's s1-s5 jacket genuinely means so much to me. Canonically, he's had that jacket since high school. It's probably full of random stains and has gross years old receipts and old scammy credit cards in the pockets. It's scuffed up. It's Dean. Sam changes jackets every season, but for the first 5, Dean keeps the same one. It's not just another layer that the boys wear all too much of. It's the one stable thing that Dean has had in his life since literal high school. Sam left, then came back. John was John, then died. He probably killed the lesbian nuns in that jacket. It's such a constant and recurring motif that we don't realize its utter importance until Dean packs it away with his impala keys when he is planning on saying yes to Michael in 5x18. He wears a different jacket for the rest of this episode, but by 5x22, Swan Song, he is wearing the Jacket again, representing his action to seperate himself from the angels, to reject the cosmic game and be Dean Winchester.
The first note for season 6 in my notes app is 'dean that is not your jacket where is your jacket'. First and foremost, the different jacket, just like how the Impala is covered and his guns are locked, represents Dean's new life-- but why does he take the Impala when he leaves Lisa and Ben but his jacket is still different? (the jacket actually changes quite a bit during this season) In my opinion, it represents a shift in the narrative. Yes, Sam and Dean are still saving people, hunting things, the family business, but the stakes have shifted. Throughout s1-s3, the 'Big Bad' has always been directly centered on the Winchesters. s1 is setup, classic spn, finding Dad. Season 2 is Azazel/revenge the demon children, Sam dying at the end, setting up for s3, which is saving Dean. S4 is a little different: it introduces Castiel, and we have brotherly tensions relating to Dean's time in hell and demon blood and Lilith, but the pacing is relatively similar to s1 and s2 (lots of classic folklore fights) s5 is back on the brothers-- yes we have the apocalypse, but the question is not how to save the world, but how to save each other for the brothers.
From season 6 onwards, the Big Bad is saving the world. We have the War in Heaven + Castiel becomign God in s6 setting up for leviathans in s7. Yes, the focus is still on winchesters + castiel, but the stakes are expanded to the whole world. They are fighting things not ever discussed in John's hunting journal or the bible. The show has gone off the rails but we are too sucked in to care. Dean's jacket is gone. His old way of hunting: saving people, hunting things, the family business doesn't exactly fit this new deus ex machina world. We stop hearing 'saving people hunting things etc.' each season/episode recap. However, Dean still turns up the collar of his jacket in a way Sam doesn't, echoing the way the collar of the Jacket was turned up in the back.
Now, I haven't finished all fifteen seasons. I'm at the start of s9. But I have heard rumours of a rusty nail and the horrors of being stuck in the narrative. I really hope that Dean will dig his Jacket out of wherever he stashed it. (probably still at Lisa's house tbh) and wear it. Feel the history and the way things have shifted.