My sense from looking at a few of the tests in this directory is that they are not about index construction or index maintenance, but rather about the query system's ability to construct and execute indexed plans. There are also a few tests for specific features mixed in, such as tests for wildcard indexes or index filters.

Since these are really query engine tests, I suggest that we migrate these tests from jstests/core/index/ into jstests/core/query/, or subdirectories of jstests/core/query/. Tests which simply build some indexes, run queries (with or without hints) and assert that those queries return the correct results can probably remain in jstests/core/query/ for now. Tests pertaining to specific query engine features such as wildcard indexes should probably be moved to appropriately named subdirectories.

Also see this related ticket about improving the naming of these tests: SERVER-96696.