It's no longer possible to spawn on scaffolding after respawning was changed in 20w30a.

How to reproduce:

1. Place a bed
2. Place scaffolding in 10 surrounding floor blocks around the bed
3. Block the space above the bed
4. Use the bed to set respawn point
5. Die (/kill) and respawn

Observed Results:

Player respawns at the default world spawning location, with the following message:

"You have no home bed or respawn anchor, or it was obstructed" 

Expected Results:

Player respawns on top of scaffolding blocks. 

Screenshots/Videos:

20w29a - Can spawn on scaffolding: https://youtu.be/Hs-VzACdcv0

21w41a - Can't spawn on scaffolding: https://youtu.be/Mhu_DVlLEOM

Code Analysis:
Code analysis by apple502j can be found in this comment.