Just purchased my Mac this past weekend and found out through support that Big Sur 11.2.3 does not support Target Display Mode with older Macs. My other Mac is running High Sierra 10.13.6. The store sold me the Thunderbolt cable and Thunderbolt 3 to Thunderbolt 2 adapter, informing me that it will work. Support went on to say that if I only bought my Mac in 2019 that I would've been able to enjoy this feature. I have already sent my feedback on the issue, but just wanted to let everyone else here know what I found out today.