The redemption of Mendicant Bias hasn't been ignored, as the legacy of his last known action may still result in what the ancilla desired. A healed (albeit, digitized) Ur-Didact recovered from the Domain's healing, hopefully setting on a journey to recover the also-missing Janus Key to aid the Reclaimers. This will be Mendicant's atonement, but only if this comes to pass. As of now, all we know is the Didact reflecting on the memories of his wife, imprisoned in the Domain. The question is, will he be freed, and if he will, by whom?
The next time we encounter the Domain might be when we see Shadow-of-Sundered-Star once more, perhaps. If Mendicant is to have survived the destruction of Installation 08, then his inactivity could be explained by awaiting the return of his makers, as he's keeping been very hush through the years. He wasn't waiting for the Reclaimers to return, or else he would be active once Mayhem
crashed on the Lesser Ark.
How else can his "masters" (this being the active Forerunners, seemingly, of which there seems to be a few, or just the Ur-Didact) know that he's changed, unless either the Master Chief tells them who sent him to Requiem, or unless they know for themselves by venturing once more to the Ark? Unless Mendicant Bias has been shelved permanently, the only way this plot thread moves is through the Didact (or whatever is going on with the Forerunners at Bastion, of which we know little of) knowing of Mendicant's "example", simply. It's a waiting game until that event happens.
How can you say his redemption hasn't been ignored with such confidence when there hasn't been any activity from him since Halo 3? Installation 00 has been visited twice now with a research fleet being stationed (at one point) yet no activity from Mendicant. We've seen a plethora of plots killed (Black Team,Janus Key) or outright ignored ( The Assembly, Reach seemingly being a shield world) most specifically the Didact himself, i think it is more than reasonable to question of Mendicant has been forsaken or will simply be written off as KIA at this point. From his monologue Didact is still far from coming to terms with Humanity as a whole and at this point returning as some uber Promethean Knight would be disengaging as Halo Reach's utter retcon of events.
The Domain has already returned via Cortana as that's what stabilized her rampancy as Mantle's Approach made an emergency slipspace jump. Per Halo 3 IRIS/ Contact Harvest Mendicant Bias was indeed waiting for contact with Humanity as this was explicitly stated in both mediums. Unfortunately this only seems to further notion of his plotline being concluded despite the massive set up established as a cliffhanger. All of Mendicant's shards were said to have been collected over the course of Halo 2-3 with the Keyship returning to Installation 00 coalescing them all.
We've known Forerunners were alive and watching since IRIS, Halo 4 Terminals merely reinforced the notion.