This root and ones like it were connected to taste throughout Tolkien’s life. The earliest manifestion of this root was ᴱ√TYAVA “to savour, taste” with derivatives like ᴱQ. tyausta “savour, flavour” and ᴱQ. tyava- “it tastes of, reminds one of” (QL/49); in the contemporaneous Gnomish Lexicon it had derivatives like G. caf- “to taste” and G. côf “savour, smack, odour” (GL/24).
In The Etymologies of the 1930s the root became ᴹ√KYAB “taste” with a derivative ᴹQ. tyav- of the same meaning (Ety/KYAB). This root and verb also appeared in the Quenya Verbal System of the 1940s (PE22/102). The root appeared again in Late Notes on Verbs from 1969, first as √KJABA “taste”, then as √KJAW “taste, select, choose”, the latter with derivatives Q. tyav- and S. caw- “taste” (PE22/151, 152).
Neo-Eldarin: For purposes of Neo-Eldarin, I recommend assuming the root was √KYAW, since some of the later derivatives like S. caw- can only be derived from this version of the root.
The root √TAS “point out, indicate” was mentioned a couple times in notes on Eldarin Hands, Fingers and Numerals from the late 1960s as the basis for various words for the index finger as a pointer, most likely as an extension of the demonstrative root √TA (VT47/11, 13, 26, 29). However, in the Qenya Lexicon of the 1910s, the root ᴱ√TASA had derivatives like ᴱQ. tasarin “willow” and ᴱQ. taste “fringe” and was connected to ᴱ√TAŘA [TAÐA] which was the basis for “hedge” words (QL/89-90). A superscript S after the root seemed to indicate the primitive form was indeed ᴱ√TASA rather than ✱✱ᴱ√TAÞA as pointed out by Gilson, Hostetter, Wynne and Smith. The words G. tast “border, fringe” and G. tathrin “willow” from the contemporaneous Gnomish Lexicon were clearly related (GL/69), the latter most likely the result of the sound change whereby sr became thr in Gnomish as it also did in later Sindarin.
The root ᴹ√TAS reappeared in revisions to The Feanorian Alphabet from the early 1940s, where it served as the basis for ᴹQ. atsa {“claw” >>} “tassel, fryse, fringe” (PE22/50). However, as the basis for ᴹQ. tasar(e) “willow” the root ᴹ√TAS was problematic, because starting in the 1930s intervocalic s > z > r in Quenya (PE19/49), as opposed to Early Qenya of the 1910s where this transition happened only before nasals, liquids and voiced spirants (PE12/15-16, 19). Thus in The Etymologies of the 1930s Tolkien introduced a new root ᴹ√TATHAR as the basis for ᴹQ. tasar(e)/N. tathor “willow-tree” and N. tathren “of willow” (Ety/TATHAR). The root {√TASĀR >>} √TATHAR reappeared in Words, Phrases and Passages in the Lord of the Rings from the late 1950s or early 1960s as the basis for Q. tasar/S. tathar “willow” (PE17/81).
Neo-Eldarin: For purposes of Neo-Eldarin, I would posit the existence of a Neo-Root ᴺ√TATH as a replacement for early ᴱ√TASA to salvage early “fringe” words as well as being the basis for √TATHAR, reserving for √TAS the sense “point(er)” it had in the late 1960s.