“In that state He also went and made a proclamation to the spirits in prison 20 who in the past were disobedient, when God patiently waited in the days of Noah while an ark was being prepared. In it a few — that is, eight people — were saved through water.” 1 Peter 3:19-20

After death on the cross Jesus went to the prison and preached to souls there. It sounds like the only souls are those who died before Noah’s ark. What was so special about the souls before Noah’s ark? Why not preach to all the souls who have died before The Cross. There is two theories. One, that Jesus went to proclaim judgement against the fallen angels who were loosed during the times of Noah and was taking the daughters of man as their wives. And was bound during the days of the flood. 

 Genesis 6:2-4 (CSB) 2 the sons of God saw that the daughters of mankind were beautiful, and they took any they chose as wives for themselves. 3 And the LORD said, "My Spirit will not remain with mankind forever, because they are corrupt. Their days will be 120 years." 4 The Nephilim were on the earth both in those days and afterward, when the sons of God came to the daughters of mankind, who bore children to them. They were the powerful men of old, the famous men.

Some people believe that these demons are held in a prison until the time of Revelation 9

Second theory is that Jesus went down to prison to preach repentance to the souls that have died and the people of Noah’s generation are an example of they type of people Jesus was preaching to.   

Chuck Smith says it this way in his cometary:

“Jesus preached to the souls in prison. Now in the prophecy concerning Jesus, in Isaiah 61, says, "The spirit of the Lord is upon me; because the LORD has anointed me to preach the good tidings to the meek; he has sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to those that are bound" (Isaiah 61:1).

What is he talking about? The prison was death by which men were bound. They were held captive. Jesus came to open the prison to those that are bound, or to open up Hades to those people who were bound there, who died before Jesus died for our sins. So when He died, He descended into hell. And He preached to those souls that were in prison. And when He ascended out of hell, He brought with Him those who had been captive.

In Ephesians chapter four, Paul tells us that "He who has ascended is the same one who first of all descended into the lower parts of the earth. And when he ascended, he led the captives from their captivity" (Ephesians 4:8-9).

Luke's gospel, the sixteenth chapter, Jesus describes what hell was like prior to His death: Two compartments, no capacity of crossing from one to the other. One was a place of torment, the other was a place of comfort. Those who died in faith went to the place of comfort and were comforted by father Abraham. They are the ones to which Jesus preached when He descended into hell. But "God did not leave his soul in hell, neither did he allow the Holy One to see corruption," but this same Jesus has God raised from the dead (Acts 2:27).

And Matthew's gospel chapter twenty-seven tells us that the graves of many of the saints were open and they were seen walking in the streets of Jerusalem after His resurrection from the dead. He led the captives from their captivity. So opening the prison.” https://www.blueletterbible.org/Comm/smith_chuck/c2000_1Pe/1Pe_003.cfm

There is more evidence for the second opinion. It is just like God to give people a second chance for repentance. The people of Noah’s day heard the saving grace of God now Jesus is going into prison to tell those who are there and it was all about Him. I think all the saints of the old testament preached to their people about what God was doing and Jesus went to prison to proclaim that what they were preaching was Him.