Target Display Mode not working on MacBook Pro 13" 2020
Same issue here! 

New 2020 MBP, using an apple USB-C to thunderbolt adapter, apple thunderbolt cable, mid 2011 iMac. 

Apple support just informed me that they don't have this feature in newer devices. What a shame!

I'm filling out a feedback form suggesting that they bring it back. Perhaps naively hopeful that they'll respond but worth a shot.