1) This workaround was successful on a Sprint-locked Pixel 6, but for SIM lock only. The "OEM unlock" toggle switch in developer options to allow fastboot to unlock bootloader is still disabled.
2) According to https://source.android.com/devices/bootloader/locking_unlocking
: "Devices should deny the fastboot flashing unlock command unless the get_unlock_ability is set to 1. If set to 0, the user needs to boot to the home screen, open the Settings > System > Developer options
menu and enable the OEM unlocking
option (which sets the get_unlock_ability to 1)." But this is not possible at the moment.
3) I found the following tidbit on an old reddit post with respect to modifying the system values that the "OEM unlock" toggle switch represents. That OP never received an answer, but his research could be an avenue to explore for this problem by someone with the right knowledge (not me).
View attachment 5572723
Sprint/T-Mobile kicked my old, but wonderful (SIM unlocked, bootloader unlocked, and rooted) HTC 10 off their new 5G network and provided a (SIM locked, bootloader locked, non-rootable) Pixel 6 to replace, which is nice, but not as nice as fully owning my phone.
Massive kudos to the entire XDA community and its contributors for all their work.