Apostate Protestantism is a phrase taken from the writings of Ellen G. White that the Seventh-Day Adventist Church now utilizes to refer to Protestant Churches who, the SDA Church claims, are daughters of the whore of Babylon—the Roman Catholic Church. This is because going to church on Sunday is supposedly the “Papal Sabbath” and is supposedly the Protestant churches vestige back to the Roman Catholic Church (not true).
This is also downstream of the Adventist Church’s teaching around “Babylon” in the book of Revelation which they claim is obviously protestant Church’s because it symbolizes doctrinal confusion. The irony of this belief is that the SDA Church itself fits the bill much better than Protestantism.
Adventist’s believe they were raised up by God at a specific time, in a specific way, for a specific purpose and that is to take a “present truth” message that them—and only them—were given by God to take to the rest of the world, including Christians. Part of this message is that one must leave their current church (which is part of Babylon) and come join them—God’s Last Day Remnant—or you will receive the Mark of the Beast (worshiping on Sunday) and receive the wrath of God in full strength. This is not typically the messaging they want to lead with, but it’s true nonetheless.
You can also watch sermons from popular Adventist Pastor’s such as John Lomacang who go into this in detail.