This seems to be the word that Tolkien favored for “daughter” in his later writings (PE17/170; VT47/10; PE19/73), though it had competition from other forms like Q. yeldë.

Conceptual Development: The earliest word resembling this form was ᴱQ. sui “daughter” in the Qenya Lexicon of the 1910s under the early root ᴱ√SUẈU (QL/87), a word also mentioned in the Poetic and Mythological Words of Eldarissa (PME/87). This became ᴱQ. silde “daughter” in Early Qenya Word-lists of the 1920s (PE16/135).

In The Etymologies of the 1930s, Tolkien experimented with several different forms. He had ᴹQ. yelde “daughter” under the root ᴹ√YEL, but this entry was deleted (Ety/YEL). Tolkien also had a root ᴹ√SEL(D) “daughter” with a derivative ᴹQ. selde, but the meaning of this root was changed to “child”, and masculine and neuter forms ᴹQ. seldo and ᴹQ. selda were added to the entry (Ety/SEL-D). Finally, under the entry for ᴹ√YŌ or YON “son”, Tolkien added a primitive feminine variant ᴹ✶yēn or yend “daughter”, producing ᴹQ. yende and (suffixal?) yen (Ety/YŌ).

These vacillations continued in later writings, where at one point Tolkien wrote “Q[uenya] Wanted: Son, Daughter” (PE17/170). In Notes on Names (NN) from 1957 Tolkien wrote Q. sel-de for “daughter”, but above it he wrote a variant form anel. In rough notes from around 1959 Tolkien explored a large number of masculine and feminine suffixes, and on the page he had yeldë “daughter”, though at the end of the sentence he wrote “also yen” (PE17/190). In notes on Eldarin Hands, Fingers and Numerals from the late 1960s, Tolkien wrote selyë as a diminutive/affectionate word for “daughter”, with seltil as a play name for the fourth finger representing a daughter (VT47/10, 27).

Also of note is Tolkien’s Quenya name for S. Tinúviel “Daughter of Twilight”, which he generally represented as something like Q. Tindómerel < ✶Tindōmiselde. Tolkien was fairly consistent in this Quenya form starting in the 1930s (Ety/SEL-D; PE19/33), with examples in the 1950s (PE19/73) and 1960s (VT47/37) as well. Indeed, in a couple cases he used this name to illustrate how medial s generally became z and eventually r in Quenya (PE19/33, 73), so it seems that for this name Tolkien consistently imagined the primitive form for “daughter” as ✶selde.

Neo-Quenya: I’d assume selde is the main word for “daughter” for purposes of Neo-Quenya, but I’d assume a variant form yeldë, especially since -iel was the most common suffix for “daughter of”. This variant probably arose very early under the influence of √YON “son”.