Depends where you look.
From a "Green Line Extension" standpoint, ever since the land-swap deal with Guilford/DelawareNorth that traded away Old Lechmere for the New Lechmere site (as part of a much bigger deal), "we all" knew that there'd be an interval where service to Old Lechmere would be bustituted, partly because the developers wanted to get their hands on the Old parcel, and partly because there was going to be a gap as the steel El structure was torn down and replaced.
From the FMCB meeting, Sec. Pollack said that the Lechmere Viaduct rehab was accelerated 4 years to coincide with the GLX-induced closure. (Had it not been, she said, they'd have faced the embarassment of opening the GLX and then 3 years later closing the viaduct.
As for why it doesn't appear in "upcoming closures" my guess is that they know that it is +/- 30 days from April 30, and specific enough to think that Union Sq will be ready "11mo to 12mo later" but not quite ready to announce a date certain for bustitution.
But we do see it deep inside.
Bid comparison for the contract
(which said it was being coordinated with the GLX's March 2020 to April 2021 closure of Lechmere)