One would expect to be able to do so due to the solid surface available on the side of the block, however this is not the case. This is different from MC-25152 as it concerns placing torches on top.
All of the blocks in the title should be able to be placed on soul sand and honey blocks, however ladders ad vines would not be able to be supported by grass paths or farmland, as they require a full block face and these blocks are only 15/16 of that. (Should I make soul sand/honey blocks and grass paths/farmland into two separate issues?)
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Torches can only be placed on the sides of solid blocks. Both farmland and grasspaths have a height of 15/16th of a full block. They are therefore considered transparent blocks since they include a transparent part (the upper 1/16 of the block). This means the behavior described in this ticket is intended.

In which case, please remove the ability to place torches on the backs of stairs.

The back of a stair fills a full block side, though. That's not really comparable.

There's still more than enough space on said surface.

Leaving it open, as for @unknown it allows placing it on any solid face, not solid block (this logic changed in 1.12).

FVbico and Connor Steppie: You are correct, my mistake. I am however interested in the point Christian Rau brought up. It would ultimately depend on what the intentions of the developers are, though...

Relates to MC-8345

Relates to MC-70057

also soul sand now

This mecanic is logic in my opinion

Yes, awful logic, and not a mechanic.

This is probably intended.