Unfortunately, there is a single alphabetical index in Writer. User-defined “indexes” behave like a table of contents, i.e. entries are listed in order of appearance in main text (not alphabetically) and therefore identical entries are not merged into a single line with a list of page numbers.
However, with some restrictions in layout of indexes you can cheat to give the illusion of several alphabetical indexes.
I have absolutely no idea of what SCOTUS format looks like. I went superficially on the mentioned link and it seems to me that the cases, statutes and misc indexes are contiguous. If this assertion holds true, then the single index can be formed with three subparts, but these parts may not be listed in the intended order.
Cheat trick no. 1
Usually, index entries are entered “as is”: the word(s) make up the key. You mau have noticed that when you Insert>TOCs &Indexes>Index entry, the dialog prompts for up to 3 keys: entry, 1st key and 2nd key. Enter your to-be-indexed term(s) into entry and the “categpry” as Cases, Statutes or Misc. into 1st key.
When you generate your index, you’ll end up with 3 main words (the “categories”) with dependent term alphabetically sorted beneath.
To format your index to your liking, modify paragraph styles Index 1 for the “categories” and Index 2 for the entries.
Cheat trick no. 2
Your categories are listed alphabetically as Cases, Misc. and Statutes. Usually, we prefer that Misc. be listed last. To force this ordering, add a space before Cases and Statutes. If your index is long enough, Misc. will be sufficiently apart from the others so that the (lack of) extra space will not be noticeable.
My workaround assumes you are familiar with styles and does not dig into details.
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