It depends on the headphone and the market. 10 years ago, the hd560s hadn’t been released yet, while now it’s one of the best for the price, if not the best, at least in the EU market, while in the USA the 6XX was released prior to the hd560s and depending on taste it’s arguably a better deal for that market. Koss 60 ohm lineup has been out for quite some time, and they’re a pretty good deal and probably we’re just as good when they were released. The hd600 has come out in 1997 and for that kind of sound it is to this day the best deal to get it. Then you have the recent hifiman that have come out in the last 10 years and are a pretty good deal and compete well with what’s available today, yet the quality control is still not up to par. I’d say the golden age for headphones happened around the 90’/2000, since most models that come up today have to make up to the good reputation of models that came out in that period. Some stax headphones came even before that, and for some specific aspects they’re still the best, even though new models have come out. I’d say there isn’t a general rule of thumb for this, it depends a lot on what is considered “good” and what price range you’re looking at. Still, some pretty good headphones have come out in the recent years, and some pretty good headphones from the last decades are still relevant today, but there’s still a lot of junk being produced today, as there was some decades ago. What’s true is that good models stay good, no matter when they came out