Two beta streams - dev, where a dev account is required ($99 per year) and the public beat which is basically a few weeks behind dev, minus early betas. The availability is always announced at WWDC, normally the dev availability is on the day of the keynote.
Much like Windows devs, Apple devs need to pay attention to this. If your livelihood is dependent on a platform, not paying attention is on you.
Microsoft does distribute beta versions of Windows for third-parties to test against.
Does apple do the same? If Apple doesn't, then it's Apple's we have to blame it onto.