Shown is a screenshot of torches placed onto the bottom halves of slabs and stairs using setblock. There is clearly enough space for the torch to be placed on the specified area of the block, however manually placing a torch on them through the inventory will not work.
Other blocks with a large enough area, such as stonecutters, sunlight sensors, end portal frames, enchantment tables and cauldrons also cannot have torches be placed upon their sides manually.
This may be considered working as intended, however given the changes to supported block placement on transparent blocks in recent snapshots I'd rather report this just to be sure.
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Can confirm in 21w05b.
Can confirm in 21w06a.

Torches cannot be placed on the sides of sculk sensors either.
Can confirm in 1.16.5 and 21w08b.
Can confirm in 21w14a.
Can confirm in 1.17.1 and 1.18 Pre-release 5.
Can confirm in 1.18.1.

Can confirm in 1.19.2.
Can confirm in 20w51a.