In protestantism are all mankind equal under God?

Your question:

In Protestantism are all mankind equal under God? Are there people of higher worth and lower worth? Is everyone deserving of equal respect?

The short answer is YES. But as Ray Butterworth and Lesley pointed out, there is a difference between worth, acceptance, and respect depending on their action and lifestyle (who can respect / accept an unrepentant robber?). So I am going to assume you're asking about committed Christians in a state of grace (Catholic) or one of the elect (Reformed) about their worth in the eyes of God.

About slavery, your question implied the 18th and 19th centuries when Christians owning slaves were socially accepted in some parts of America and in colonial India (under England rule). It was EVEN MORE acceptable in the 1st century AD in the Roman period, the period of Jesus and St. Paul, "where 85-90% of the inhabitants of Rome and peninsula Italy were slaves or of slave origin" (Article on Slave, Slavery from the IVP Dictionary of Paul and His Letters). Therefore it's instructive to hear how St. Paul's commented about slaves, equality and respect (taken from a paragraph of the Theological Significance section of the article on Paul's Letter to Philemon in the IVP Dictionary of Paul and His Letters):

Second, it provides a small commentary on slavery in the ancient world. When read together with Colossians 3:22-4:1, we begin to appreciate how conversion to the Christian faith broke down all social, racial and economic barriers (Patzia, 91–93). Although Paul does not speak directly for the abolition of slavery, this letter exemplifies, as much as any other writing of his, the truth of Galatians 3:28 : “There is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus” (NIV). A new relationship and partnership has been formed in this situation where master, slave and apostle are all part of one family in Christ (Philemon 16). The church as a whole should be characterized by such virtues as love, forgiveness, equality and fellowship.

It's obvious how counter cultural Christianity was in the time of St. Paul, although social realities prohibited the direct abolition of slavery. Therefore St. Paul instead advocated that if both slaves and masters are members of the body of Christ, they should regard each other as brothers and sisters in Christ, equal in worth and respect because both master and slave were SLAVES in comparison to Christ who bought them both with His blood. So it is very important to distinguish social reality and spiritual reality.

The same distinction goes with the different social station assigned to men and women in Paul's time Ephesians 5:21-33 where Paul taught for husbands and wives to submit to one another because both need to submit to Christ. Equality in spiritual status does not necessarily require equality in social status.

Paul also dealt with racial attitudes between the Jews and the Gentiles. Jews despised Gentiles because Gentiles were usually sexually immoral, eat pork (unclean food), worship idols, and not possess the Law (revelation from God). Gentiles despised the Jews because of their separatism (not intermarry) and refusal to participate in some important civic function (such as worshiping the city's God or joining the military). Hence Paul wrote Romans 9-11 to show how in faith God called both Jews and Gentiles into the Kingdom of God.

It turns out throughout the 2000 year history, various eras and denominations dealt with slavery differently (see very extensive wikipedia article Christian Views on Slavery). An example of how a pro-slavery pastor misused of the Bible (and then refuted) can be read here.

Back to our own time, slavery is now thankfully taboo for good reasons. Finally in a Christian society the social reality now matches the spiritual reality. But alas, the society is becoming increasingly non Christians, so losing the benefit of solid grounding of equality in the eyes of God, replaced by pure power struggle in identity politics and class conflicts.

We are all related

“And he made from one man every nation of mankind to live on all the face of the earth, having determined allotted periods and the boundaries of their dwelling place,” ‭‭Acts‬ ‭17:26‬ ‭

Granted we were split up because of our rebellion at the Tower of Babel and God decided to start His own nation from Abraham Deu 32:9, through Isaac and then Jacob.

That one man was Adam and in him being equal, equally fell with him.

“Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned—” ‭‭Romans‬ ‭5:12‬ ‭

Now when it comes to salvation the nation God made, namely Israel was meant to have better access to Him that they would be the vehicle to redeeming the other nations back to God but the nations could still reach out to God if they wanted to.

“that they (nations) should seek God, and perhaps feel their way toward him and find him. Yet he is actually not far from each one of us,” ‭‭Acts‬ ‭17:27‬ ‭

But after the resurrection of Jesus, the Old Covenant which operated by the Law was made obsolete and all including the Jews could come into covenant by faith John 1:13 but they were still trying to earn their salvation by works Romans 9:32.

Ultimately whosoever means everyone is equally welcome to believe on Jesus and be saved, thereby becoming chosen or elected for the kingdom if they remain IN Him, Jesus. Moving out of Adam and into Christ the second Adam.

We are all equal whether saved or unsaved in the eyes of God. And are responsible for our own individual decisions concerning our salvation.

With regards to differences, those are dependent on decisions with consequences. All decisions have consequences and these can reach out far beyond the immediate sphere of one, out even as far as generations upon generations. But value wise we are all equal under God, with unalienable rights common to all men.