The thief was forgiven by Christ as he forgave others while alive, under the OT! Baptism wasn't a requirement of salvation until Christ died, at which poitgd NT began. Read Hebrews 9:16-17
16For where a testament is, there must also of necessity be the DEATH of the testator. 17For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth. It's a false argument trying to relate the thief on the cross with baptism.
You are right about the New Covenant not being in effect until AFTER Christ had died. What John preached was intended to clear the way for that which would soon come to pass. God had revealed to John, that during his time, the Messiah would appear personally and announce to all people"here I am" and that He Himself would be the embodiment of the Kingdom:
6Therefore My people WILL KNOW My NAME; therefore they will know ON THAT DAY that I am He who speaks. ...Here I am!”
7How beautiful on the mountains are the feet of those who bring good news, who proclaim peace, who bring good tidings, who proclaim salvation, who say to Zion, “Your God reigns!” (Isaiah 52:6-7)
The way to enter into the Messiah's Kingdom would be by believing in Him which meant repenting of our sins and surrendering our lives to Him personally. Could people who trusted in the Messiah before be saved even before He appeared and completed His work on the cross? Apparently so, otherwise the Bible would not have said
5 When Jesus saw their faith, He said to the paralytic, “Son, your sins are forgiven.” (Mark 2:5; Matthew 9:6, Luke 5;20).
Included in this, was the command to enter into covenant with the King by being water baptized. While the water itself does not sanctify us following through with this is an ACT OF OBEDIENCE to the gospel. The bottom line is that is the express the will of God for all who would be His.
At the same time we do not seem to want to go into the waters. At least, when I came to Christ, I REALLY did not want to confess Him before men. Walking to the front of the Church felt like walking against a powerful wind. Each step of that journey seemed to be death to who I was. After that, the Holy Spirit gave me another command- be baptized. This was just as hard. When I was about to go into the Baptismal water I began to feel something else - my heart melting in His love. He had wanted me to go through that not to humiliate me but to separate me from the things that would have killed me. Forgiveness swept all pollution away and united me to Him.