it doesnt hurt to know. just differentiate both x and y variables, but every time you differentiate y just multiply by dy/dx then make dy/dx the subject. be careful tho because if its something like x^2 + y^2 =4, you need to differentiate both sides, so the 4 becomes 0. it might be helpful to bring eveything to one side first.
Technically they’re not in this syllabus in the “explicit” sense. So it is unlikely you will find HSC questions in the current syllabus explicitly saying “use implicit differentiation...”.
However, the idea of differentiating both sides and using the chain rule is not a concept outside the syllabus so you may need to apply it to solve a problem - though I would expect that to be a rare occurrence or come with guidance attached.