What is the difference between Feed The Beast Legacy Packs, and Feed The Beast regular? There is http://legacy.feed-the-beast.com/modpacks/ with a different set of packs than https://www.feed-the-beast.com/modpacks . What is the difference?
Legacy is the old, discontinued version of the FTB Launcher. They have stopped developing it and switched to Curse instead, so Curse will have newer modpacks and more 3rd Party Packs, while the FTB launcher will only have older packs. Though I heard that if you want to run older packs, the legacy launcher will be better.
The address you mentioned is copy of old layout. Legacy site offers some of the old content. It should not be used at all unless you are seeking some specific information which is not moved to new website.
All FTB packs are available through Curse client and FTB Launcher. There still is no plans to stop distributing FTB packs only with Curse because some features which are present in FTB Launcher are missing from Curse.