It is introduced in later Delphi versions and adopted by Freepascal. Note that untyped consts are always signed, though. That has not changed a single bit.
Delphi has very new versions that are free for personal use, btw.
So don't make comparisons between Freepascal and Delphi unless you are current.
Properly phrased inconsistencies between the two compilers will always have the attention of the compiler developers. (the reverse is also true: Delphi developers follow Freepascal)