Here in the printer’s manuscript, Oliver Cowdery initially wrote “& it came to pass that after the space of many years”. Later, probably when he proofed 𝓟 against 𝓞, he crossed out the that (the crossout was done with somewhat heavier ink flow). Since either reading is possible here, there would have been no motivation for Oliver to make this correction except to make sure that his copy followed the reading in 𝓞 (which is not extant here but probably read without the that ). For further discussion of Oliver’s tendency to add that after “it came to pass”, see under 3 Nephi 8:5. For each case of that after “it came to pass”, we follow the earliest textual sources, thus the corrected reading here in Ether 10:9.
Summary: Maintain in Ether 10:9 the phrase “and it came to pass” without the following that, the corrected reading in 𝓟.