While not a "Mastodon client", Smithereen does use it to recursively load complete reply threads when you open a post using the search or when someone boosts a remote post at you.

There's also a problem: if there's a deleted reply somewhere in the thread, it will make the other part of the thread unreachable (up or down, depending on where you started traversing from). Smithereen solves this by not deleting such replies completely, but instead serving a Tombstone object that still has `id`, `replies` and `inReplyTo` fields.