Step 1:Generate a raid

Step 2:Teleport the captain to the nether or far enough away so that it will leave the raid 

Step 3:Throw a raid flag to a pillager in raid

You can see that the pillager will not put on the flag or be the captain although there's no captain in the raid 

Expected:The pillager puts on the flag and be a new captain

By the way,whether the captain that leaved the raid was killed or not,there will not be a new captain anymore