To be honest, it's not so much a "legacy app" issue as a "legacy programmer" issue.
I started programming back in Ruby 1.8.6. I don't use any opinionated web development frameworks such as Rails (I don't do web development at all, actually!), so nothing told me to switch to minitest. I used 'require "test/unit"' back then, and I still use it now. I don't actually know whether I use test/unit or whether Ruby 1.9/2.0 uses minitest with a compatibility layer. I also don't know what advantages there are in switching from test/unit to minitest.
When I switched from 1.9.x to 2.0, the way that tests were outputted changed, in that you usually don't get a series of dots being printed out for each test. That gave me the impression that whatever I was using was non-legacy.
Is test/unit considered legacy?
I think that documenting the advantages of minitest over test/unit, or that test/unit is considered legacy, may be helpful. Maybe in the documentation for test/unit?