Actually, UEFI firmware is not the only one that is required for BitLocker. The system BIOS can also be used by BitLocker.
For BitLocker to use the system comprehensive check provided by TPM, the computer must have TPM 1.2 or later versions. And a computer with TPM must have a Trusted Computer Group (TCG) which is compliant with BIOS or UEFI.
The BIOS or UEFI establishes a chain of trust for the pre-operating system startup, and it must include support for TCG-specified Static Root of Trust Measurement.
But TPM 2.0 is not supported in the Legacy and Compatibility Support Module ( CSM) modes of the BIOS. So devices with TPM 2.0 should have their BIOS mode configured as UEFI only. And at the same time, disable the Legace and CSM.
So, provided it is TCG compliant, the BIOS is also supported. But not all TPM chips support the Legacy BIOS, and you will need the UFEI firmware to use BitLocker on a later version of Windows.
For more about BitLocker, you can also read:
How to Find BitLocker Recovery Key And Do BitLocker Recovery?
How to Unlock BitLocker Drive Without Password & Recovery Key?