Do any Protestants teach that everyone really knows God according to Romans 1:21? For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. 
  (Romans 1:21, English Standard Version, ESV, Emphasis mine)


Are there Protestant denominations that teach that everyone knows God to some extent or do they teach that Paul is referring to Jews or Christians only?
Yes, many Christian's do teach this verse. Because it is in the Bible. One example of a such a teaching i will include as a link. It is a sermon by Pastor John MacArthur ( A well known protestant teacher). 

https://www.gty.org/library/sermons-library/45-11/reasons-for-the-wrath-of-god-part-2

This verse is often taught in the sense that everyone has an inner knowledge that there is A GOD. Not that everyone personally "knows God", in the sense which we can come to know the Lord Jesus Christ.

An example can be seen when we think of mankind's natural instinct to create and worship Gods. Even though men usually come up with the wrong idea about God (idolatry). This can be seen clearly in Pauls encounter with the Athenians. (Acts 17:22-31 included) (maybe someone can edit this and help me format the bible quote. I have forgotten how to do new lines.)


  
    22 Then Paul stood in the midst of the [i]Areopagus and said, “Men of Athens, I perceive that in all things you are very religious; 23 for as I was passing through and considering the objects of your worship, I even found an altar with this inscription:
    TO THE UNKNOWN GOD.
    Therefore, the One whom you worship without knowing, Him I proclaim to you: 24 God, who made the world and everything in it, since He is Lord of heaven and earth, does not dwell in temples made with hands.
     25 Nor is He worshiped with men’s hands, as though He needed anything, since He gives to all life, breath, and all things. 26 And He has made from one [j]blood every nation of men to dwell on all the face of the earth, and has determined their preappointed times and the boundaries of their dwellings, 27 so that they should seek the Lord, in the hope that they might grope for Him and find Him, though He is not far from each one of us; 28 for in Him we live and move and have our being, as also some of your own poets have said, ‘For we are also His offspring.’ 29 Therefore, since we are the offspring of God, we ought not to think that the Divine Nature is like gold or silver or stone, something shaped by art and man’s devising. 30 Truly, these times of ignorance God overlooked, but now commands all men everywhere to repent, 31 because He has appointed a day on which He will judge the world in righteousness by the Man whom He has ordained. He has given assurance of this to all by raising Him from the dead.”
  


In light of this portion of Romans, Paul is also teaching that the rejection or this inner knowledge of God, is enough that every man will be held accountable when God judges the world, with no one being able to say "But i didn't know there was a God!" 

Hope this is a helpful introduction to a very interesting portion of scripture!