What is the yellow marking underneath Delta A220 doors?
This is almost certainly an alignment marking for the passenger boarding bridge (jetway). You can find the same markings on other Delta aircraft as well:


(image sources: A320, B737, B767)

The dead giveaway is that these markers are found on the second door on the long haul jets, which are typically boarded through this door instead of the first one:


(image sources: A330, B777)

The jetways used by Delta will have a similar marking on them which aids in properly aligning them with the aircraft door such that the door can still be opened and passengers can safely board and disembark.

See also: What are these markings under the passenger door?