Nature Letters has historically been a "safety valve" for scientific publication in that it provided a way for a Nature Editor to allow the publication of short and possibly controversial pieces that might potentially be important, but that would find it difficult or impossible to pass normal peer-review.
In October 2019 Nature announced that the suspension of the Letters section was now a permanent feature of the journal's new redesign.
This is, obviously, the end of an era. Was the format redundant? Did the science community not produce enough interesting but controversial material to keep it going? Are scientists today more conservative than they used to be and less willing to take risks with their reputations? Does this leave the scientific community without a respected "official" channel for communicating controversial ideas and results?