Can confirm this is still the case in 1.16.5 and 1.17-RC1
This bug seems to still be an issue in 1.16 even though the bug that it is a duplicate of has been resolved in 1.15
Can confirm all of these do not support transparency in 20w21a
Paintings
Item frames
Beacon beam
Book on the enchanting table
Conduit (when not 'powered' lacks transparency, but when part of a completed conduit multiblock, it has transparency)
Trident (entity form)
Confirmed to still lack transparency in snapshot 20w06a-20w15a:
Paintings
Item frames
Beacon beam
Book on the enchanting table
Signs can be transparent again 🙂
The enchanting table book also no longer supports transparency, and should be added to the list.
this also happens with the arrows in a written book, even though a writable book doesn't have the issue.