As discussed under 3 Nephi 8:5, evidence from scribal and typesetting errors argues that the subordinate conjunction that was probably accidentally added here in Mormon 1:8 by scribe 2 of 𝓟 when he copied the text from 𝓞 into 𝓟. The 1830 typesetter seems to have omitted the that only when he was setting the text from Isaiah and the King James Isaiah lacked the that. For a more general discussion, including the evidence from transmission errors, see under 3 Nephi 1:22.
Elsewhere the text prefers that between “it came to pass” and the adverbial phrase “in this year”; there are seven instances with that and two without (each of the latter is marked below with an asterisk):
Here in Mormon 1:8, it would appear that scribe 2 of 𝓟 accidentally added the that because he expected it.
Summary: Retain in Mormon 1:8 the 1830 reading without the that after “and it came to pass”; it is more likely here that scribe 2 of 𝓟 supplied the that than it was omitted by the 1830 typesetter.