This is the first Star Trek episode to directly address if the Trill are members of the Federation, confirming that they are not as of 3189, though it is unclear if they were members from the 2200s to the 2300s, when the bulk of Star Trek is set.
At one point, Saru addressed Paul as "Dr. Stamets." While not necessarily an error, as Paul does hold at least one doctorate, no one has ever addressed him that way before. He is always referred to either by name or rank.
In addition to Admiral Senna Tal, at least one other prior host of the Tal symbiont wears an older Starfleet uniform. Another appears to wear a uniform similar to the Bajoran Militia (the overwhelming majority of the militia are Bajorans, but they also allow non-Bajorans, such as Odo, Rom, and Kurn).
This episode features the first trans-trans kiss in Star Trek franchise history, between the non-binary actor Blu del Barrio (Adira) and trans-man actor Ian Alexander (Grey). Because of their gender-fluid nature, the Trill race have been embraced as representation by the trans community for decades.