The City has been planning this for years if not decades--this presentation will be three years old in just a few days (and imagine how many months and years prior to it, the City was working on it, even allowing for any COVID-related interruptions):
https://www.boston.gov/sites/default/files/file/2021/10/DTX Public Presentation#1 09-22-2021.pdf
That said, it's hard for me to conceive of a more difficult area to execute technically-feasible design plans, given how DTX is absolutely honeycombed with hollow vaulting beneath the sidewalks PLUS the Orange Line tube running the length of Washington AND the Red Line tube running the length of Winter/Summer.
Coordination with the (presumably) dozens of different property owners who have those hollow vaults, MBTA, Eversource, Veolia, all the telecomm providers, and the collage of easements they all have stacked-up within any cross-section of the area? It won't be easy--but hopefully it gets done at some point in the next few years...