There is strong evidence that in polysyllables, any final diphthongs ending in i were reduced to a long ī in Sindarin and Noldorin, and this long vowel was later lost with various phonetic effects. This sound change has been noted by David Salo (GS/§4.33), and is needed to explain the development of Sindarin plurals, which regularly demonstrate i-affection and i-intrusion, such as S. erain plural of aran “king”. There are a few explicit examples of this plural development in Sindarin and Noldorin:

• -ai > -ī: ✶phelgai > felʒi > S. fili, plural of S. fela “minor excavation” (NM/304).
• -āi > -ī: ✶tekmāi > tekmī > tiŋw > S. tīw, plural of S. têw “letter” (PE17/43).
• -ōi > -ī: ᴹ✶góndōi > gondŏi > ON. gondī, plural of ON. gondo “stone” (PE21/58).

There was a similar change of final i-diphthongs to [ī] in Welsh, also influencing the forms of plurals in that language (WGHC/§75viii). In theory this sound change might only apply to plurals, but we also have an explicit example of this phonetic development for a word that is not a plural:

• -ai > -ī: ✶Ossai > ossī > ussi > S. †yssı̯, an archaic Sindarin cognate of Q. Ossë, later called S. Gaerys (WJ/400).

This example (as well as the plural examples above) indicate the reduction of i-diphthongs was a general sound change, not specific to plurals, and probably occurred at an ancient stage of Sindarin’s history. However, some Telerin plurals indicate this changed did not occur in that branch of the language: Elloi, plural of T. Ello “Elf” (WJ/373); Lindai, plural of T. Linda “Singer” (WJ/382). Thus, this change probably did not date back to the Ancient Telerin period.

There are examples indicating this sound change did not occur in monosyllables, such as: √MAY > mai > S. mae “well” (PE17/17, 162). Also note that in Noldorin, it seems this sound change did not occur for the final diphthong -ui [-ŭi] as indicated by a number of Old Noldorin plurals:

• ᴹ✶orku > ON. orko, plural orkui > N. orch “Orc”, archaic plural yrchy, later yrch (Ety/ÓROK, EtyAC/ÓROK).
• ᴹ✶ranku > ON. ranko, plural rankui > N. rhanc “arm”, archaic plural rhengy, later rhenc (Ety/RAK).
• ᴹ✶smalu > ON. (s)malo, plural malui > N. (h)mâl “pollen”, plural mely vs. [more regular] meil (Ety/SMAL, EtyAC/SMAL).

These Noldorin developments were pointed out by Bertrand Bellet in his article on Noldorin Plurals in the Etymologies (NPE), where he suggested the final -ui became a monophthong -y but was generally reformed into a more regular plural, as in the examples above. There is no sign of any retention of primitive -ui in Sindarin; most notably it doesn’t appear in any of the later etymologies of S. orch.

Many of these ancient i-diphthongs were long diphthongs such as -āi and -ōi. The likely mechanism for this change is that these first became short diphthongs (as with góndōi > gondŏi), then the initial sound in the diphthong became less distinct [ǝ] and ultimately -ǝi > -ī. A hint of the latter development occurs in a discussion of tengwar representations of Old Noldorin from the late 1930s:

The chief exceptions were [final] ī in standard ON (from ǝı), long and distinct from ĭ; also ū occurred finally distinct from ŭ (PE22/27).

It is an open question whether this sound change also applied to final u-diphthongs in polysyllables; this last note hints that it might, but there are no clear examples. Unlike the plural, u-diphthong would not be the result of ancient dual nouns, since the dual ū replaced the final vowel, so if such a change occurred, it would be very rare.

Conceptual Development: It is likely that Tolkien introduced this phonetic rule into Early Noldorin at the same time he introduced plural forms using i-affection. He mentions this sound change in the discussion of plural forms in the Early Noldorin Grammar:

Mutations [in plurals] are really epenthesis of ı̯ from older ī < ī, ai, oi &c. (PE13/121).

As noted above, the primitive diphthong -ui seems to survive in Noldorin to later become -y, but this was often reformed, especially in plurals. There is no indication that there was a similar ui-survival in Sindarin.

See OS. final i-diphthongs became long [ī] in polysyllables for discussion.