The R5ii and R1 sport Canon's new flash mount. Does anyone know anything about these mounts and the new Canon Speedlites?
OK, I don't think I'm going to write a massive essay here, but heres the short version.
Starting in the fall of 2021, Canon introduced the Multifunction shoe (MFS) concept on every new product*. Starting with the R3, cameras had the 21 contacts of the MFS built into the front edge of the hotshoe in addition the the conventional five contacts (four Canon-specific ones behind the universal center contact). Along with the R3, they introduced the AD-E1 adapter, ST-E10 flash controller, and DM-E1D microphone as the first MFS accessories. As time went on they introduced the EL-5 as the MFS replacement for the 600EX line and the EL-10 as the MFS replacement for the 430EX line.
Cameras - except as noted - have both the MFS contacts and the 5-contact arrangement (think of it as a combo shoe), but the accessories only have one or the other. To adapt 5-pin accessories to the MFS, the AD-E1 is used. However, it's rarely actually needed. Adapting takes two forms: one is the conversion from 5-pin to 21-contact MFS, which is only needed on the R50 as it's the only camera without the 5-contact arrangement, and the other is adapting the weather-sealed hotshoe gasket of the premium flashes to the different shape of the area surrounding the new, larger hotshoes. And in this case, people have reported that the older flashes with the gasket will still squoosh into place on the newer bodies.
Bottom line is you can generally use your older flashes with the newer bodies. However, there is no way to use the newer MFS flashes with cameras lacking the 21 MFS contacts, which is every camera predating the R3 including the R, RP, R5, R6, and all DSLRs.
* The R50 has only the MFS contacts and the R100 bizarrely has only the traditional 5 contacts. All of the others have both, including the upcoming R5ii and R1.