But you need to balance having "wastage" against being unable to fullfil demand.
The "out of date" shirts will always sell and if you discount by 20% the club will stay make a profit albeit at a far lower margin. If you can't meet demand, you take no money and make no profit at all.
It's "O"...
https://www.footyheadlines.com/2023/07/full-breakdown-price-of-football-shirt.html
This would suggest that where the club retails directly to fans (as opposed to selling through a sports shop chain) the margin for the club is just under 40%.
If the club didn't change the design of all the kit each year, there wouldn't be a risk of having tons of "out of date" stock. Just change every 3/4 seasons and when you hit the end of the cycle, you have a sale.