I learnt about 'oblique' asymptotes at school and they even examined it in assessments. It hasn't been explicitly stated in the syllabus because it just says "asymptotes" in general. However, one of the examples recommended in the syllabus has an oblique asymptote of y = x:
From section 10.5 of the 2 Unit part of 'Geometrical Applications of Differentiation":
"The sketching of curves such as quadratics, cubics and higher polynomials and simple rational functions. After computing some values (which may include points where x = 0 and where y = 0), the determination of the stationary points is frequently very useful. Other considerations are symmetry about the axes, behaviour for very large positive and negative values of x, and the points at which functions such as: y= 1/(x - 1), y = x + 1/x, y = 1/x, are defined"
The curve y = x + 1/x has an asymptote of y = x, and this curve has been stated as an example in the 2 unit part of the syllabus. So from that I guess you can say that oblique asymptotes are actually in syllabus, and more surprisingly the 2 unit syllabus....