In UM1472 rev.5, Table 6. STM32 pin description versus board functions, the Free I/O column as it is now is dysfunctional.
It has an interesting history: the pins respective to individual functions (including free) used to be marked by highlighted (greyed) background in the given table cell, in rev.1. In rev.2, empty cells got filled by "-" marks, including the greyed-out cells which were in fact used. Then, somewhere along the line to rev.5, the highlights disappeared.
Please mark clearly the Free I/O pins in that table.