Not sure if that was a question, but I’m referring to the fact that the R5 C has the multifunction shoe, but it doesn’t work with the ST-E 10 transmitter or the EL-5 flash (nor with the new EL-10, presumably). Other accessories (e.g., Tascam audio inputs) that work in the R5 C’s shoe work on other multifunction shoe bodies, but Canon’s own flash doesn’t work on the R5 C…and that almost certainly means it’s just a firmware issue.
Since the R5 C is a hybrid camera that boots into a stills mode, it would make sense for Canon’s new flashes to be supported. Users on Canon’s forum have certainly requested it, but compatibility wasn’t listed in the recent R5 C firmware update (the first in a while, but there have been updates since the ST-E 10/EL-5 came out). Shame on Canon for not addressing this.