It isn't called THE departure. I thought I mentioned that back several posts ago.
Apparently you didn't make your case well enough.
Literal Standard Version
do not let anyone deceive you in any way, because if the
departure may not come first, the man of lawlessness may [not] be revealed—the son of destruction,
Geneva Bible of 1587
Let no man deceiue you by any meanes: for that day shall not come, except there come a
departing first, and that that man of sinne be disclosed, euen the sonne of perdition,
Coverdale Bible of 1535
Let noman disceaue you by eny meanes. For the LORDE commeth not, excepte the
departynge come first, and that that Man of synne be opened, euen the sonne of perdicion,
Tyndale Bible of 1526
Let no ma deceave you by eny meanes for the lorde commeth not excepte ther come a
departynge fyrst and that that synfnll man be opened ye sonne of perdicion
World English Bible
Let no one deceive you in any way. For it will not be, unless the
departure comes first, and the man of sin is revealed, the son of destruction,
1539 Cramer Bible
Let no man deceaue you by eny meanes, for the Lorde shall not come excepte ther come a
departynge fyrst, and that that synfull man be opened, the sonne of perdicyon,
Same is true with the 1384 Wycliffe Bible as well as the 1576 Breeches Bible, 1583 Beeze Bible.
From what I understand even Jerome in the 4th century translation of the Latin Vulgate used the latin word for departure.