Apparently the original manuscript had forbid as the past-tense form for the verb forbid here in 3 Nephi 26:11 (since both the 1830 edition and the printer’s manuscript read this way). Such usage is found in commonly spoken English (including my own), as in the example “he forbid me to go”. The Oxford English Dictionary lists this past-tense form as dating from Early Modern English (beginning in the 1500s) and continuing up to the 1800s. For the third impression (in 1907) of the 1905 LDS edition, the colloquial forbid was replaced with the standard forbade.
The parallel passage in 3 Nephi 28:25 has forbade in both the 1830 edition and the printer’s manuscript:
So in this second case the original manuscript apparently had forbade. Thus the original manuscript seems to have had some variation between forbid and forbade as the past-tense form for the verb forbid. Of course, it is quite possible that in 3 Nephi 26:11 the use of the colloquial pasttense form forbid was an error on the part of the scribe in 𝓞, presumably Oliver Cowdery.
These two examples are the only ones of the simple past-tense form for the verb forbid in the whole Book of Mormon text. It is perhaps worth noting that for the related verb bid, we have only bade as the simple past-tense form (and without variation throughout the entire textual history):
Like the case of forbid, my own English prefers bid in all these past-tense instances. We should also point out the one case of past-participial bid that occurred originally in Helaman 5:49; this form implies the theoretical existence of the simple past-tense bid (see the discussion under that passage). The past-tense form forbid is therefore quite possible as the original reading here in 3 Nephi 26:11, although it could also be an error due to dialectal speech. In any event, the critical text will restore forbid since that form is possible.
Another possibility worth noting is that the use of “the Lord forbid it” in 3 Nephi 26:11 could be an error for “the Lord forbiddeth it” (or even “the Lord forbids it”)—that is, perhaps the verb forbid is in the present tense. Note that later in the text, in Ether 13:13, we have such a present-tense passive construction involving forbid: “and I was about to write more but I am forbidden”. Nonetheless, 3 Nephi 26:11 is syntactically similar to 3 Nephi 28:25, which definitely has the past-tense form forbade (although not the direct object pronoun it):
David Calabro (personal communication) suggests that 3 Nephi 26:11 could be a mistake for “but the Lord forbidded”. Here Oliver Cowdery could have misheard Joseph Smith’s dictated forbidded as forbid it —that is, the unstressed -ed ending was misinterpreted as the direct object pronoun it, also unstressed. If this suggested emendation is correct, then the simple past-tense form is a regular past-tense form derived from the base form of the verb forbid. And this would mean that there would be no it after the verb, just as in 3 Nephi 28:25 where no direct object is stated for forbade.
One problem with this proposal, however, is that there is no evidence elsewhere in the text for such a misinterpretation in the dictation of the text. Moreover, the earliest reading, forbid it, will work since the dialectal past-tense form forbid does exist. The critical text will therefore accept the earliest reading for the verb form here in 3 Nephi 26:11.
Summary: Restore in 3 Nephi 26:11 the original past-tense form forbid for the verb forbid in place of the standard past-tense form forbade; such usage is supported by usage in Early Modern English as well as in today’s colloquial English.