Here are three instances in the original text (the first of which is extant in 𝓞) where the pastparticipial form for the verb forget is forgat, not forgot or forgotten . The 1830 compositor replaced each of these instances of forgat with forgot. Of course, in standard English the past participle is forgotten, which the 1906 LDS large-print edition introduced into the LDS text. The 1907 LDS vest-pocket edition adopted the first two of these emendations but accidentally left the third one as forgot. From 1911 on, the LDS text has consistently read forgotten for this passage.
In the original text, there are many instances where the past-participial form is the same as the simple past-tense form. (See, for instance, the example of had came under 1 Nephi 5:1, 4. For a general discussion, see under past participle in volume 3.) In the original Book of Mormon text, both forgat and forgot are acceptable as the simple past-tense forms for forget; and thus both are also acceptable as past-participial forms. We therefore follow the earliest textual sources here in Alma 60:20, the reading of the manuscripts, forgat. For evidence that the original text had instances of both forgat and forgot as the simple-past tense form (although usually forgot), see under Alma 37:41.
Summary: Restore in Alma 60:20 the original three instances of the past-participial form forgat, which is morphologically equivalent to the archaic simple past-tense form forgat.