Word was you'd still be able to download copies you'd purchased previously - any problems now I'd assume are snafu's that will be fixed. The implication was that iPhoto and Aperture will run at least through Yosemite (and hopefully at least the next release) while we get some bug fixes and updates on Photos.
I realized I had forgotten to buy the newer (bluish icon version of iPhoto that had dropped features from the prior version of iPhoto, so I avoided it) from the App Store and tried to do so on launch day, but couldn't pull it up.
Best idea is to find a copy of the installer now while they are still somewhat plentiful (making reminder to myself) if you want iPhoto working on Yosemite while 1.0 Photos matures.
Basically the OS will refuse iPhoto Installation after the next update or so. This happened with my version of Final Cut Pro and Aperture already, but not with iPhoto yet
If you go to 'Purchased' in Appstore on OSX El Capitan. you can try to download it, but it will say there is an error (because the file no longer exists and is deleted from server). The same happens with iphoto, aperture, tomb raider underground (not available anymore in appstore and your purchased list does not allow it to download)
I have lost; Tomb Raider Underworld, OSX Server, Final Cut Pro, Aperture.
You wont be able to download it from purchased since the file is currently deleted.
The purchased list is hence now completely erroneous, and Apple are suable for theft of digital property that is purchased.