As above, history of art is an entirely separate department - single honours history students aren't able to take HoA papers either.
The history syllabus is changing for those starting in 2021 and onwards, and the form of the outline/topic papers will be different from the current papers. From their description on the website, the outline papers will include various forms and approaches to history including political, economic, and social history, whereas currently those are divided for British history papers. More info here:
https://www.hist.cam.ac.uk/prospective-undergrads/history-course/course-structure-from-2021In any case, current HisPol students not only aren't restricted purely to political history papers, they in fact cannot take more than a single paper in British political history anyway (and aren't required to take any papers in British political history). At least one paper must be taken in the history of political thought in the current HisPol tripos, but the history of political thought is
not political history. You probably will not have studied the history of political thought previously. You can see examination regulations for that tripos at present in the Statutes & Ordinances here:
https://www.admin.cam.ac.uk/univ/so/2019/chapter04-section19.htmlGenerally speaking though for
any degree there is no guarantee any particular optional paper/module will be available in future and optional courses do in fact often change, as staff leave, go on sabbatical, or engage in research fellowships, and of course staff may just get bored of teaching a particular paper and teach a different one. It is impossible to say whether any specific optional papers available to current students will still be available in three years. It is likely
some will still be in situ, but it is also likely some won't be, and there is now way to know which will stay and which will go.