A), you can generally find a place at many urban destinations to donate clothing.
B), lots of us have clothing that we don’t hate but are ready to be done with, for one reason or another. For instance, I generally pack my raggediest old underpants when travelling (sorry Mom!). They make it through one final wearing on the trip, and then get thrown in the trash, which is probably less resource-intensive than washing and re-transporting them anyway.
Bonus, if the loss of those raggedy garments has depleted my usable underwear supply too much, I can get some nice foreign undies as a souvenir of my trip! (No I do not show them off to guests, very funny.)
The One Super Useful Weird Tip for long-stay travel packing, though, is: Thrift stores at your destination. I left for a six-months overseas trip last year with one modest half-filled suitcase containing a few key garments and the abovementioned ratty underwear. And for less than US$100, I obtained a lavish temporary wardrobe (including shoes) that I then cleaned and re-donated before returning home.
Think of it as just a rental fee for having all the clothes you need without either the hassle of massive packing, or the environmental costs of transporting massive amounts of luggage.