Hi George
Don't worry about the word "Legacy" and just use the functionality. It's not the only place that "legacy" is used in Photoshop.
My best guess is the thought process at Adobe goes something like this: " Based on user feedback (and software engineer whims), Adobe has decided to make the UI easier and more delightful for new users and old users alike (except the grumpy old ones that don't like change 
). For the grumpy users, we will keep the old functionality and mark it as "Legacy" to match the old and grumpy users Old grumpy users like me
OK that was tongue and cheek (sort of). There is a lot of history with Photoshop and a lot of older scripts and actions that depend on the functionality to remain the same. While these new features don't keep the scripts and actions exactly backwards compatible, with a few changes they will still operate the way they should.
In some cases it means Adobe believes they have a better way to meet customer needs and/or better UI (easier to use). Some functionality does get tossed from Photoshop once in a while. Sometimes something takes its place and sometimes its gone forever. In general, they are pretty good about keeping old features around even if buried down a bit more.
I guess Adobe assumed that it was better to keep the little used slices function (even if accessed down one more level in the menu and marked Legacy) rather than losing customers that still use that functionality.
I would just use it and not worry about it. If you are somewhat worried, write into Adobe and let them know that you do not want that functionality to go away in future releases. I have no indication that is the case yet Adobe is not all that transparent with their plans.
Just my opinion of course
John Wheeler
