The original mission/scope appeared in Historia Mathematica in 1978 (vol.5(1), p.76). The text below is a large citation from this journal:

There were International Congresses on Mathematical Instruction in 1968, 1972 and 1976. At the 1972 Congress, held in Exeter, England, there was a “working group” (EWG II) concerned with the relations between the history and the teaching of mathematics. The activities of this group were continued at the
1976 Congress in Karlsruhe. […] The Executive Committee of ICMI has now approved the affiliation of this Study Group under the title, “International Study Group on Relations Between
History and Pedagogy of Mathematics, cooperating with the International Commission of Mathematical Instruction”.

The principal aims of this Study Group are as follows:
1. To promote international contacts and exchange information concerning: (a) Courses in History of Mathematics in Universities, Colleges and Schools. (b) The use and relevance of History of Mathematics in mathematics teaching. (c) Views on the relation between History of Mathematics and Mathematical
Education at all levels.
2. To promote and stimulate interdisciplinary investigation by bringing together all those interested, particularly mathematicians, historians of mathematics, teachers, social scientists and other users of mathematics.
3. To further a deeper understanding of the way mathematics evolves, and the forces which contribute to this evolution.
4. To relate the teaching of mathematics and the history of mathematics teaching to the development of mathematics in ways which assist the improvement of instruction and the development
of curricula.
5. To produce materials which can be used by teachers of mathematics to provide perspectives and to further the critical discussion of the teaching of mathematics.
6. To facilitate access to materials in the history of mathematics and related areas.
7. To promote awareness of the relevance of the history of mathematics for mathematics teaching in mathematicians and teachers.
8. To promote awareness of the history of mathematics as a significant part of the development of cultures.