No, there are no BTF AM4 motherboards available.
No, there are no BTF AM4 motherboards available.
No, you could only test one motherboard. BTF is not a standard, it's a design concept. Msi, Asus, and Gigabyte all put stuff in different places and no boards 100% line up, so unless that changes there won't be a test bench like that. Plus when you think about the clips, they'd be a nightmare to deal with. Either the board won't clip in and you can possibly have a loose power connection (look at 12vhpwr for how that goes) or it will be a nightmare to remove the board.
It makes no sense unless there was a standard for high power GPUs to take their power from the motherboard. Even then, you're just adding more points of failure as you have an extra connection from the motherboard to the GPU, rather than taking power straight into the GPU from the PSU. This is backwards considering the whole point of having the connector on the GPU was to not have to over-engineer something onto the motherboard to provide more power than the PCIe slots can. So you have an extra power plane on the motherboard and a socket, taking away space for more important things like M.2 slots or chipsets. This makes motherboards more expensive and with less M.2 slots, just to hide the cables. Then if the power connector does fail, it potentially takes out your GPU and motherboard, not just the GPU and PSU plug. ATX has stuck around for a good reason, it gets the job done in the most practical and cost effective way. It would be outright stupid to switch to BTF as standard, it has far too many drawbacks.