Quenya only allowed five final consonants: n, t, l, r, s (Let/425; PE19/104; PE22/62; VT42/7), that is only dental nasals, voiceless stops, liquids and s. Where labial and velar nasals and voiceless stops became final, these final consonants became dentals. However, this begs the question of what happened to other types of consonants, namely semi-vowels as well as voiced and voiceless spirants. Final isolated voiced stops were not an issue, since in Ancient Quenya voiced stops became spirants.

Final semi-vowels simply vocalized and formed diphthongs, as mentioned in the Outline of Phonology (OP2) from the 1950s:

y, w becoming i, u formed final diphthongs, for the treatment of which see the Vowels (OP2: PE19/103).

It is conceivable the spirants were dentalized finally, but I suspect this was not the case, at least not universally. Most likely in those rare cases where the velar spirant ʒ derived from g became final, then this [ɣ] from [g] vanished as it did elsewhere, though finding explicit examples is difficult. The only likely case seems to be: ✶-wēg(o) > wē > Q. -we (PE21/81). Likewise it is likely that final -ƀ or -v became -u̯, a sound change mentioned in the Qenya Phonology of the 1910s (PE12/24) and the Qenya Phonology of the 1920s (PE14/68), but there are no Middle or Late Period examples.

The development of final -ð is known, however: it became -r as it did medially between vowels. However, this sound change was earlier than the general change of ð > r since it occurred in both the Ñoldorin and Vanyarin dialects (intervocalic [ð] became [z] in Vanyarin):

d > ř > r, as in CE tad “thither”, PQ tar ... N.B. here ř > r in Vanyarin (and not z) (OP2: PE19/104).

That leaves only the aspirates which became voiceless spirants. In OP2, Tolkien said that a final th ([tʰ] or [θ]) became t:

[Ancient final] th became t and as such remained ... when vowels were lost after other consonants or after groups, as happened in later AQ and PQ under certain conditions (e.g. especially in long compound words), these consonants or groups were changed or reduced to one of these permitted dentals: ... th, þ > t (OP2: PE19/104).

Tolkien did go on to say that in some cases s was substituted for the final -t in the Ñoldorin dialect:

But in TQ s was substituted for t < th, after the change of þ > s, on the analogy of the treatment of groups containing ancient s. t remained in Vanyarin dialect the normal final representative of þ (OP2: PE19/104).

However, there is only one attested example of -th > -t, and it appears beside a variant with -s:

• ✶Morñoþŏ [> *Morñoþ] > Q. Morñot/Morños (PE19/81).

Indeed, most of the attested developments show -th > -s (presumably via -þ):

• √LOTH > Q. lōs (PE17/26, 160).
• ✶ñwa-lōth > Q. olos (PE17/160).
• ᴹ✶Tulkathŏ > ᴹQ. Tulkas (Ety/TULUK).
• [deleted] ᴹ√MBAT(H) > ᴹQ. umbas (EtyAC/MBAT(H)).

Thus it seems -th > -t is a rule that Tolkien described but didn’t actually use.

As for final -ph and -kh, these seem to be very rare: Tolkien said that “no certain case of final p, ph, b is cited (OP2: PE19/102-3)”. What may have happened to ᵽ, f and χ, h in those (likely very rare) cases where they became final after short final vowel losses is unclear. They may have dentalized and become -t or -s. Alternately, they may have merged with -ƀ, -ʒ to become u̯ or vanish, respectively.

Conceptual Development: In the Qenya Phonology of the 1910s, Tolkien said final voiceless spirants were voiced, and then ʒ vanished, w̑ > u and þ > đ > z > r (PE12/20). For example: ᴱ✶eχt·taþ· > ᴱQ. ektar “hawthorn” (GL/18, QL/35). The same basic developments are seen in the Qenya Phonology of the 1920s (PE14/68).

Tolkien did not discuss these phonetic developments in detail in the Outline of Phonetic Development (OP1), aside from d [> ð] > -r (PE19/52). Thus it isn’t clear when Tolkien transitioned from the system of the 1920s to that of the 1950s.