Here we have a probable instance of a scribal slip, where Oliver Cowdery accidentally wrote art rather than the correct are. The 1830 compositor set the correct are. As noted under Mosiah 2:24, there are four cases in the manuscripts where Oliver initially wrote a correct are and then accidentally crossed the e to form a t. However, here in Mosiah 15:16, the t of the art looks like an actual t rather than a crossed e. Even so, the art is probably a scribal error. One possibility is that in 𝓞 (no longer extant here), the scribe (possibly Oliver himself ) accidentally crossed the e of an original are, giving art, which Oliver then copied as art into 𝓟. Creating a t for this passage would have been facilitated by the fact that both the immediately preceding and following words have t ’s, which could have prompted the scribe in 𝓞 to miswrite “that are still” as “that art still”. Since there is considerable evidence for a scribal tendency to miswrite are as art, the critical text will assume here in Mosiah 15:16 that the 1830 emendation to are is the original reading.
Summary: Accept the 1830 emendation of art to are in Mosiah 15:16 (“that are still publishing peace”).