After the introduction of a large upgrade package for Spain, McLaren continued to make minor changes for the next race in Monaco, and the most obvious change is the addition of T-wing in front of the rear wing.
The T-wing, at the height of the old rear wing, which was before 2017 at 950 mm height, has been banned since 2018, as well as large fins that longitudinally split the engine cover, but the loophole has remained at some extent.
That is why teams can still use the 750 mm wide T-wing at the height below the rear wing to produce additional rear downforce and improve the airflow quality to the rear wing.
Some teams used such T-wing at the start of the season, and some decided to add it for certain tracks, such as McLaren did in Monaco.
McLaren’s MCL33 still has a top speed deficit so adding T-wing would only make it worse, but as the top speed is not so important in Monaco, McLaren has decided to add its own version of T-wing.