Any other character following a backslash is taken literally. Thus, to include a backslash character, write two backslashes (\\). Also, a single quote can be included in an escape string by writing \', in addition to the normal way of ''.
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The link you provided goes to the wrong subsection. The following subsection, which discusses, String Constants With C-Style Escapes, requires that you write the literal as E'abc\'def'
Note the E prefix on the literal, which is the thing that enables considering backslash as an escape.
This hasn't changed from 9.6, has it?
A Java app that uses backslash escapes broke this morning on fields with single quotes, after the weekend migration from PG 9.6.24 to 14.12, and I don't know why. I'm not a Java programmer, though.