I would like to know if there are sources on the history of the classification of mathematical subjects. Gérard Lang
Here is one such historical overview:
Mathematics in library subject classification systems, by Craig Fraser (2016). (Springer link)
Insofar as library science is concerned, modern classification of mathematical subjects occurred within the larger framework of library classification, a vast project receiving sustained attention in the period from 1870 to 1920. The work of the library cataloguers was carried out against the background of a broad nineteenth-century interest in the classification of knowledge. We explore different views during this period concerning the position of mathematics in the overall scheme of knowledge, the scope of mathematics, and the internal organization of the different parts of mathematics. We examine how mathematical books were classified, from the most general level down to the level of particular subject areas in analysis. The focus is on the Library of Congress classification system in its various iterations from 1905 to the present.