How will our people learn Reformed doctrine in church now that synod has removed the requirement for catechism sermons?

You are correct that Synod 2022 deleted Church Order Article 54-b, which said, “At one of the services each Lord’s Day, the minister shall ordinarily preach the Word as summarized in the creeds and confessions of the church, especially the Heidelberg Catechism.” Synod made that change at the same time that it deleted the words “ordinarily twice” from Article 51, which says, “The congregation shall assemble for worship on the Lord’s Day to hear God’s Word, to receive the sacraments, to engage in praise and prayer, and to present gifts of gratitude.” One could argue that it made sense to eliminate 54-b when churches would no longer be required to have two services on Sunday, and therefore couldn’t make that second service the traditional “catechism service.” 

However, Article 54-a remains in the Church Order and says, “The proclamation of the Word shall be central to the worship of the church and shall be guided by the creeds and confessions.” So it doesn’t matter if your church has only one Sunday service because all sermons should be guided by the creeds and confessions. And even if synod’s requirement isn’t as specific as it used to be, ministers should continue to base their preaching on the creeds and confessions so that the congregation will indeed learn the church’s doctrine! The confessions, especially the Heidelberg Catechism, are helpful for worship planning, offering guidance for sermon series on the Lord’s Prayer, the Apostles’ Creed, and the Ten Commandments.  Moreover, following the catechism like a lectionary will encourage preaching that covers the full counsel of God’s Word, not only the preacher’s favorite passages and topics.

One church planter I know spent his first year preaching through the Heidelberg Catechism, starting with Q&A 1—“What is your only comfort in life and in death?”—and drew on the theme of “comfort” throughout the year. The small group was blessed and grew in numbers and in the grace and knowledge of Jesus Christ. Thanks be to God!