When you say Acts 21 is no longer possible, are you answering yes to my clarifying question? I just want to understand where you are coming from.
Historical context: occurs after Jesus resurrection with primarily cultural Jews around the 40’s ad. James, the brother of Christ, is the head of the Jerusalem church which is composed of primarily cultural Jews that have embraced Jesus as their messiah but still retain the law because the temple is the center of their culture and activities. They are not willing to give that up. Paul comes to visit with some gentiles to bring tithes to the church. The Jews believe that Paul is teaching other Jews to forsake the law and that he invited a gentile into the temple which would have defiled the temple. The Jews try to kill Paul and the centurion saves him when Paul reveals his Roman citizenship.
Physical context: Paul, along with eight gentiles arrive in Jerusalem. They are welcomed initially and go to James house were the elders were also present. The next day Paul purifies himself at one of the purification pools and goes to the temple were there was a large crowd and they seize Paul. They take him out of the temple and shut the doors so that the temple would not be defiled. They beat Paul outside of the temple among the large crowd until the Roman soldiers arrive so this is still outside of the temple. The soldiers took him toward the barracks with the crowd following where Paul addresses the crowd outside of the barracks.
Cultural context: there is a lot to cover here but I’m going to just summarize. Jerusalem is the center of life for the Jews including the temple were they would congregate along with other Jews from other cities that would travel to Jerusalem to worship or attend special days or festivals. This was the center of cultural Israel. The Jews were cultural people whose life revolved around the Torah and the Law. The law had been given to Moses to give to Israel because they were God’s chosen people so it made them unique among the rest of the people inhabiting the Lavant. The Jews considered preaching against the law to be a capital offense so James had to thread lightly when teaching the Jewish converts about salvation by faith without the works of the law.
On the other hand, the churches that Paul planted were primarily gentile that also included Jews. Paul could teach salvation by faith without the treat of death but with opposing groups of Jewish converts promoting the law. Apparently the problem was particularly problematic in the Galatian church. The rest of the Apostles that left Jerusalem were also able to teach salvation by faith. BTW- based on accounts by early church fathers James was eventually martyred (stoned) for his Christian believes.
Situational context: the events of Acts 21:27-40 happened because of the clash of Christian doctrine of salvation by faith apart from the works of the law and the converted Jews that refused to give up what their ancestors had believed for 15 centuries.
A proper exegesis of all the elements of context would not be possible in this platform since it would take many pages to unpack it but this will do as a summary. As you can see the cultural Jews in Jerusalem still attended all events in the Temple and the Temple still ruled their lives. The law, among the Jews, continued for his reason. James had to be careful of what and how he preached the gospel of good news. A few years later, and prophesied by Jesus in Matt. 24, the temple will be destroyed and the Jews would be expelled from Jerusalem ending the temple life and dispersing the Jews.
The events of Acts 21 cannot happen again because all if its context can not be repeated. The cultural Jew in Jerusalem and the temple are no longer there. The law among the gentiles was never a problem because the apostles ruled in the council of Jerusalem that they are not under the law. The problem of continuing under the law was a problem unique to the converted Jews which Paul argues against in his epistles to the churches.
I hope this helps and clears my position. You do with this as you will. I have stated my case.