This is easy to reproduce – get a composter, a solid block and some seeds. Slap the block down on top of the composter and start clicking on the side of it with seeds – you can hear and see that it starts to fill.
Farmer villagers are also able to fill composters from the sides when the top is occluded.
You still cannot feed anything into the side of a composter using a hopper.
(If this was added as a feature somewhere, I guess I missed it. Sorry about that. I did search first.
Comments 2
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This is absolutely a "Works as Intended", it just like you putting stuff in the chest at the bottom.