To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.
Only by reading this verse in isolation to the context can one possibly claim that it can be used to teach sola scriptura. In vv.16-19, we see that "Law and Testimony" refer to any divine mandate given by God to His people:
Bind up the testimony, seal the law among my disciples. And I will wait upon the Lord, that hideth his face from the house of Jacob, and I will look for him. Behold, I and the children whom the Lord hath given me are for signs and for wonders in Israel from the Lord of hosts, which dwelleth in mount Zion. And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? Fr the living to the dead?
In context, Isaiah is not speaking of the formal sufficiency of Scripture, but God's revelations versus that of the occult.
Furthermore, it should be noted that the word "testimony" (Hebrew: תְּעוּדָה) and its cognate terms does not always refer to inscripturated revelation. תְּעוּדָה only appears three times in the OT. In Ruth 4:7, we read:
Now this was the manner in former time in Israel concerning redeeming and concerning changing for to confirm all things; a man plucked off his shoe, and gave it to his neighbour: this was a testimony in Israel.
According to Ludwig Koehler and Walter Baumgartner, The Hebrew and Aramaic Lexicon of the Old Testament (5 vols; Brill, 1994-2000), perhaps the leading lexicon of Biblical Hebrew and Aramaic, we have the following definition of תּוֹרָה as it appears in Isa 8:20:
—2. instruction, decision from different sources, or rather from different authorities: a) from Zion Is 23 Mi 42.
—b) from the prophets Is 110 (on which see Wildberger BK 10: 36, cf. also under 4b), 524 816.20 parallel with ) תְּעוּדָהon vs.16 cf. TOB note(, 309 Zech 712.
—c) from the servant of God Is 424, cf. 4221 (gloss), 514, see Westermann Jes. (ATD 19) 190ff.
—d) from a judicial or priestly official Dt 1711, see THAT 2: 1035f.
In addition, it would be appropriate to examine all the other instances of תּוֹרָה as found in the book of Isaiah (the following verses are from the 1985 JPS Tanakh):
Hear the word of the Lord, You chieftains of Sodom; Give ear to our God's instruction (תּוֹרָה), You folk of Gomorrah! (Isa 1:10)
And the many peoples shall go and say: "Come, Let us go up to the Mount of the Lord, To the House of the God of Jacob, That He may instruct us in His ways and that we may walk in His paths." For instruction (תּוֹרָה) shall come forth from Zion, The Word of the Lord from Jerusalem. (Isa 2:3)
Assuredly, as straw is consumed by a tongue of fire and hay shrivels as it burns, their stock shall become like rot, and their buds shall blow away like dust. For they have rejected the instruction (תּוֹרָה) of the Lord of Hosts, spurned the word of the Holy One of Israel. (Isa 5:24)
Bind up the message, seal the instruction (תּוֹרָה) with My disciples. (Isa 8:16)
For the earth was defiled under its inhabitants; because they transgressed teachings (תּוֹרָה), violated laws, broke the ancient covenant. (Isa 24:5)
For it is a rebellious people, faithless children, children who refused to heed the instruction (תּוֹרָה) of the Lord. (Isa 30:9)
He shall not grow dim or be bruised till he has established the true way on earth; and the coastlands shall await his teaching (תּוֹרָה). (Isa 42:4)
The Lord desires His servant's vindication, that he may magnify and glorify His teaching (תּוֹרָה). (Isa 42:21)
Who was it gave Jacob over to despoilment and Israel to plunderers? Surely, the Lord against whom they sinned in whose ways they would not walk and whose teaching (תּוֹרָה) they would not obey. (Isa 42:24)
Hearken to me, my people, and give ear to me, o my nation, for teaching (תּוֹרָה) shall go forth from me, my way for the light of peoples. In a moment I will bring it. (Isa 51:4)
Listen to me, you who care for the right, O people who lay my instruction (תּוֹרָה) to heart! Fear not the insults of men, and be not dismayed at their jeers. (Isa 51:7)
It is clear that Isa 8:20 is not exegetically-sound evidence for the Protestant doctrine of Sola Scripura as Isaiah is clearly not teaching the formal sufficiency of inscripturated revelation, something that would actually disprove the concept of tota scriptura, an essential “building block” of the doctrine as sola scriptura requires the inscripturation of “all” (defined as being both the Old and New Testament books) divine revelation!