It seems that the new versions of Forge for 1.8.9 (tested in 11.15.0.1710 and 11.15.0.1715, without any mods) do not properly check whether there is a transparent block above the chest (or, perhaps, stairs and slabs are treated as solid blocks). In vanilla 1.8.9 chests can be accessed if there is a stair or a slab above them.

 

Interestingly enough, in this version of Forge, a chest opens properly when there is a glass block above it, which makes me think that there is an issue with slabs and stairs being treated as solid blocks by chests, but I can't be sure, of course.

 

Also, hoppers do not feed items into chests if there is a stair or a slab above them, just as if it were a solid block above the chest.

Thanks, I haven't realized it was intentional.