(This warning was retired after v0.7.2 due to low signal-to-noise ratio)
var='This is long \ piece of text'
var='This is a long '\ 'piece of text'
You have a single-quoted string containing a backslash followed by a linefeed (newline). Unlike double-quotes or unquoted strings, this has no special meaning. The string will contain a literal backslash and a linefeed.
If you wanted to break the line but not add a linefeed to the string, stop the single quote, break the line, and reopen it. This is demonstrated in the correct code.
If you wanted to break the line and also include the linefeed as a literal, you don't need a backslash:
var='This is a multi-line string with an embedded linefeed'
If you do want a string containing a literal backslash+linefeed combo, such as with
sed, you can ignore this warning.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.