This word is attested in later writings as an element in the names Methed-en-Glad “End of the Wood” and possibly Methedras “Last Peak” (of the Misty Mountains). The latter name first appeared in Lord of the Rings drafts from the 1940s as N. Methen Amon and Methendol (TI/404), making it likely that methed is a revision from the earlier adjective N. methen (Ety/MET).
This new form likely changed from an adjective to a noun, since -ed/-ad is usually a gerundal suffix in Sindarin (forming nouns from verbs). This word is clearly a noun in the name Methed-en-Glad, and could also be a noun in Methedras (= “Peak of the End?”).
The noun N. meth “end” appeared in The Etymologies of the 1930s, derived from ᴹ✶metta under the root ᴹ√MET of the same meaning and followed by an adjective form N. methen that was also glossed “end” (Ety/MET). This word later appeared as an element in the name Rochon Methestel “Rider of the Last Hope” (UT/313). It is therefore possible that meth shifted from a noun to an adjective, but its Quenya equivalent metta “ending, end” remained a noun in Tolkien’s later writings.
Neo-Sindarin: For purposes of Neo-Sindarin, I would avoid meth as an independent element, and would use the unambiguous noun form methed and adjective forms medui or [N.] methen. If you do use meth, you might use it as either an adjective or a noun, though it is more likely to be a noun.