> Each team can then prove it is backdoored by providing (a significant part of) the plaintext.

Some backdoors are asymmetric backdoor, so the original designers can decrypt anything at will.

But some others only hide a trick that reduces the computation complexity to, say, 2^80. Unless you are a nation-state, you cannot use the backdoor. And the other type of backdoor only increases the susceptibility to implementation vulnerabilities (cache-timing attacks, invalid curve attacks), mathematically speaking they're still secure, but too difficult for most people to implement it perfectly.

A escrow process would be better.