Were the Antioch believers under the Law before the Judaizers came?

The Antioch believers were not under the Law before the Judaizers came. They had become believers of Paul's gospel and were not under Law or legalism. The Judaizers were the ones attempting to put them under the Law, as the context indicates.

Les explains that when Peter refers to "disciples" in chapter 15, he is not speaking of the Twelve but of the believers in Antioch. These Gentiles had already become believers of Paul's gospel, and they were not under Law or legalism. The Judaizers and Jewish leaders, including Peter and the eleven, were coming down hard on Paul for teaching grace to these Gentiles, trying to convince him that circumcision and Law-keeping were required. Paul adamantly refused. Later, Peter realizes that years earlier in Caesarea, God had used him to take the Word to Cornelius, a Gentile, and Peter witnessed that Gentile's salvation. Peter then asks why they should test God by putting a yoke on the neck of the disciples—the Antioch believers. Les teaches that the Judaizers were the ones attempting to put these believers under the Law, but they had not been under it before.

Acts 15:10

Now therefore why tempt ye God, to put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear?

What was the yoke the Judaizers wanted to put on the Antioch believers who trusted Paul's gospel?

The yoke was circumcision and Law-keeping—legalism—that the Judaizers sought to impose on the Gentile believers at Antioch who had trusted Paul's gospel of grace. Peter called it a yoke on the neck of the disciples, meaning those Antioch believers, not the Twelve.