Sprint might be. At least they used to back when OG Air was released, iirc.
AT&T iPads will accept pre-activated physical SIM cards from other carriers but will not allow signing up for a new plan with another carrier via Settings - Cellular. The models with Apple Embedded SIM, the embedded SIM will only show plans for AT&T. These are technically unlocked and going through the automated device unlock page will tell you that the device is already unlocked. The problem is due to Carrier Visbility / Carrier Reveal policy and it's a bit of a crapshoot getting a CSR that can help with that. Might be the same situation with eSIM but I can't confirm.
Verizon iPads will accept pre-activated physical SIM cards from other carriers but will not allow signing up for a new plan with another carrier via Settings - Cellular. The models with Apple Embedded SIM have the embedded SIM completely disabled. The ones with eSIM, the eSIM will only work with Verizon (tried QR code for GigSky and it wouldn't accept it).