<Usual caveat that Chief Delphi in general is not and I in particular am not an official source>
As I read it, the key item here is the last sentence of G15, shown in bold:
Originally Posted by 2018-2019 FTC Game Manual
<G15> Robots Deliberately Detaching Parts – Parts may not be deliberately detached from Robots during a Match or leave mechanisms on the Playing Field unless permitted by a Game Specific Rule. The consequence of deliberately detaching a part is a Minor Penalty if it does not Block an opposing Alliance Robot, Alliance-specific Scoring Element or Scoring Area. If a deliberately-detached component or mechanism affects game play by any Robot, the offending Robot will receive a Major Penalty and will be issued a Yellow Card. Robot parts that are released but remain connected by a tether are considered detached for the purposes of this rule.
Originally Posted by Orange Box
Tethered components that move independent of the main Robot are considered a detached component and are illegal.
A hook is clearly a "robot part". Intentionally detaching it on a string or similar not-even-close-to-rigid tether is against the rules "unless permitted by a Game Specific Rule". Remember, the boxes are intended to clarify the actual rules - in this case the rule answers the question directly, and the box is apparently for emphasis.
If that hook were on, say, a telescoping antenna or tool handle which was at least nominally rigid? I'd guess it was legal, but would definitely check Q&A or have a plan B to put in place at competition were I planning to do it.