Are serious medical issues that resulted in multiple absences and downward trend (only one semester on the transcript) considered as “hooks?”
No. What you describe is an obstacle you had to overcome, which is often a supplemental essay question.
I’m a purist, so to me a hook is: you were either born with insane athletic ability/as a URM/into a family that has legacy and/or donates a lot of money to the school and/or are celebrities.
Agree, that is an obstacle not a hook. A person with a hook fulfills an institutional need of the college or university – be it a recruited athlete, a child of a huge donor, a famous person who might bring positive publicity etc. A person who overcame a serious medical condition does not fulfill an institutional need of any college.
Your illness might make for a good essay topic if done well (avoid the “poor me” aspect and focus on how it changed you as a person). If your illness led to lower grades for a period of time ask your guidance counselor to address that in his/her letter or recommendation to colleges.