Ok this is exciting, I called around locally bikes owned bikes shops and asked if they had some scrap parts laying around. One of them had a Shimano SLX m7000 left brake assembly. I paid for the part and disassembled the lever. It is an exact fit. So, you can take a blade from the m7000 hydraulic series and put it on the newest m8100 hydraulic series! Fits like a glove.
KEEP all the parts from the existing brake handle! Use the existing spring, newer pin, newer retention screw, and newer rubber plug/bung!
Reassembling is tricky. I made a temporary paper pin to hold the assembling together. Then I put the assembly into place, then used the real metal pin to push out the paper pin.