The missing cores (dolphin, citra, ppsspp, etc.) are a result of our server and buildbot infrastructure update/migration. They’ll return soon (no ETA but shouldn’t be too long).
In the meantime, you can download some of them from https://github.com/hunterk/libretro_builds. Here’s a build of PPSSPP from a trusted backup: https://cdn.discordapp.com/attachments/469974542299955210/797782022532759560/ppsspp_libretro.zip