It doesn’t appear cut and dried. The new hotshoe has all the connections that the old one has.
And the ‘new’ accessories including the adaptor don’t use those connections at all, and only use the new front connections.
The adaptor also boasts that it’s the only way to provide a perfectly watertight seal. But there’s every chance that whatever you sit on top of it won’t have a watertight seal.
What do Canon say in the manual?
The manual says
"When attaching accessories that communicate through contacts of the multi-function shoe, insert the accessory's mounting foot until it clicks into place, then slide the -mounting foot lockin lever to secure it. For details, refer to the accesso 's Instruction Manual.
The following accessories cannot be attached direct! to the multi-function shoe.
• Speedlite EL-1/600EXII-RT/600EX-RT/580EX II
• Speedlite Transmitter ST-E3-RT (Ver. 2)/ST-E3-Rl1
• Off-Camera Shoe Cord OC-E3
To use the accessories listed above with the camera, you will need Multi-Function Shoe Adapter AD-E1, sold separately. For details, refer to the AD-E1 Instruction Manual.
When attaching accessories other than these that are designed for regular hot shoes, insert the accessory's mounting foot all the way in, then slide the mounting foot locking lever to secure it. For details, refer to the accessory's Instruction Manual."
I have godox flashes and controllers and the off-camera shoe chord. There are a few comments about potential damage to the new front part mounting directly so I think I'll bite the bullet and get the adapter, if I can find somewhere with them in stock.