what are the topics we should be skipping/not relevant to the current syllabus?
Commenting just from reading the paper, not checking answers / methods in detail:
2013 paper: I would not worry about Q8, plane geometry is now only considered as prior knowledge. Q11(e) and Q12(a) is no longer in the syllabus. Q13(b) part (i) is possible to ask, though unlikely, and part (ii) is outside the syllabus. Q13(c) is possible but unlikely (as both 13(b) and 13(c) use concepts that are still in the syllabus, but greatly de-emphasised). Q13(d) is possible as a trigonometry exercise but the extent to which trig will be explored in plane geometry contexts is unclear. Q15(a) I wouldn't worry about (plane geometry). Some very nice questions here, but challenging.
2014 paper: Q2 is outside the syllabus, sigma notation is now MX1. Q4 is outside the new syllabus. Q11(d) us outside the syllabus as written, but inside it if the sigma notation was replaced. Q11(e) and 11(g) are outside the new syllabus. Q12(b) and 12(c) are outside the new syllabus. Q13(b) is possible but unlikely as it uses concepts that are still in the syllabus, but greatly de-emphasised. Q15(d) and (f) are outside the new syllabus, though 15(e) is doable if you assume the result from 15(d). I would not worry about Q16(c) as it is plane geometry. Again, some very nice questions here, but seriously challenging in places.
2015 paper: I would not worry about Q2. Q4 is still in the syllabus, even though limits are basically gone otherwise. Q5 uses heavily de-emphasised materials. Q9 is outside the new syllabus. Q11(d) requires the difference of two cubes, which appears rarely. Q12(c)(ii), volumes by revolution, is now only MX1 content. Q12(d)(i) could be asked but (ii) could not... and there is not much point asking (i) on its own. 13(b) is in the de-emphasised materials category. Q13(c) is plane geometry and so unlikely. The rest are ok, though some very difficult.