greeting="hello
target="world"

greeting="hello"
target="world"

The first line is missing a quote.

ShellCheck warns when it detects multi-line double quoted, single quoted or backticked strings when the character that follows it looks out of place (and gives a companion warning SC1079 at that spot).

If you do want a multiline variable, just make sure the character after it is a quote, space or line feed.

var='multiline
'value

can be rewritten for readability and to remove the warning:

var='multiline
value'

As always `..` should be rewritten to $(..).