Targum Yonasan indeed translates it as 'Forever' and R Avraham ibn Ezra as 'Truth'.
However, as @J. W. Tanner commented, Radak writes that it is a musical signifier (cf Septuagint and Vulgate).
Artcroll is presumably agreeing with the latter interpretation and therefore leaves it as it is—a hebrew musical term (as does the KJV).
(Radak, Tehillim, 3:3
Selah – The majority of commentators say of the word Selah that it is equivalent to לעולם ("for ever"); and so Jonathan renders it in the Prayer of Habakkuk (iii. 3, 9, 13) for ever (לעלמין). And in this sense the word is common in our prayers. And the learned Rabbi Abraham ben Ezra – may the memory of the just be blessed! – has interpreted (ad loc.) that it is equivalent to אמת (truth). For myself, I say it is not a significant word. And it is to be interpreted as having the meaning "lifting up" (of the musical accompaniment) from (the passage) "cast up, cast up (סלו) the way" (Is. Ixii. 10), signifying that at the point where this word is mentioned and read there was a raising of the sound of the music (מזמור). And the proof is in the fact that it is not found except in this book and in the Prayer of Habakkuk, which was a song (accompanied by music), as is written also there (iii. 19), "For the chief musician on my stringed instruments.")
Ditto if you accept Hoil Moshe's suggestion that it is a way of referring to HaShem.
(Hoil Moshe, ibid, 3:9:
Selah — Many interpretations have been offered for this word (see Otzar HaShorashim by Rabbi Wolf Heidenheim), and it seems to me that the most accurate meaning of the term, according to tradition (its translation as "forever"), is the most correct one. We only find it in Psalms and two or three times in the prayer of the prophet Habakkuk, which was composed according to the shigyonot, meaning in the style of songs known as shigyon (see what I will write, with God's help, on the meaning of the word shigyon, at the beginning of Psalm 7). Certainly, the songs of David were never forgotten by the people of Israel, not even for a single generation, and the meaning of this word has been preserved for us correctly through tradition.
If we investigate its origin, we find its pattern in the word herah ("flee" in Genesis 12:8), which is similar to "ascending" (lehar, "to the mountain"). Selah is from the root that appears in "You are still exalted (mistolel) among my people" (Exodus 9:17) and "Extol Him who rides upon the heavens" (Psalm 68:5), meaning to exalt and lift high (even though, in truth, the lamed [in salu] ought to be emphasized). Therefore, the basic application of the word Selah is as a reference to God, blessed be He. Just as we find that the word netzach is a reference to Him (and also netzach Yisrael, 1 Samuel 15:29) and is also used as a verbal form meaning "forever," the same applies to the word Selah. Examine Meir Natib, and you will find that in every instance it can be explained with one of these two meanings. None of the other interpretations offered in the Otzar HaShorashim are valid in all cases without significant difficulty.)
