I know you can't use a drywall anchor. Hopefully someone with experience can help you with the plaster wall. Don't do anything else until you have good advice.

I'd adjust the plan to work with studs wherever they are. I've had no real luck making other kinds of anchors stand up to much weight or torque.

How big are the shelves and what are you putting on them? I’ve had luck with command brand picture hanging strips on very light shelves (holding only a few light decor items) on plaster walls (those shelves had a back to attach the strips to). Command also makes picture-ledges and display ledges. They only hold 2-5 lbs, though.

The shelves are maybe just over a foot wide, but they're heavy wood shelves with heavy duty metal brackets, and weigh at least 5 lbs each, just the shelf. I'd like to put some miscellaneous items of varying weights on them. Each shelf will probably be 10+ lbs when all is said and done. There are 2 brackets on each shelf. I can't use a stud. It's a narrow space between a corner and a window and there is only 1 stud about centered in the space. I can't move the shelves far enough to the left or right to hit the stud. I'm wondering if this is a lost cause and I'm not going to be able to hang these...

I would find somewhere else to put the shelves. They really should be in studs.

Can you put at least one anchor into a stud?

Forget the shelves on that wall hang some art there and get a freestanding unit if you have to display other stuff.

With 3 cm gap behind the plaster, use Mollie/Toggles... first cut the bolts to a length equal to the 3 Cm plus the plaster thickness plus the thickness of the bracket bolt hole....

To answer the question of what would happen if you use drywall anchors, this is what happens:

Unbeknownst to us, the shelving in the pantry had been incorrectly and sloppily installed by the previous owner and it spontaneously collapsed in the middle of the night not long after we moved in.

All the shelving was held up by puny little drywall anchors (some wrapped in tape and then poked into the wall wtf!!) in thick plaster. We did load it up pretty heavily, but did so under the assumption the shelves were properly mounted (we absolutely should not have made this assumption). We got a standing shelf as a replacement.

If you can't mount the shelving properly for any reason, go with a standing shelf and don't mess around with incorrect bolts/anchors.

Thanks everyone for your input. I have decided not to hang the shelves here. I'll put them in the basement where there is drywall, and do something else with this awkward stretch of plaster wall, something light weight.

"Unbeknownst to us, the shelving in the pantry had been incorrectly and sloppily installed by the previous owner..."

Where did you see anyone suggest incorrect and sloppy work? lol

Wait what? The bad installation wasn't suggested (??), we directly observed it after the shelves had collapsed. The collapse revealed it had been held up with nothing but tiny drywall anchors that didn't even penetrate all the way through the plaster. It wasn't at all obvious when the shelving was intact, though.

actually you wrote:

"To answer the question of what would happen if you use drywall anchors, this is what happens:

"Unbeknownst to us, the shelving in the pantry had been incorrectly and sloppily installed by the previous owner..."

The point might be made that the suggestion that "if you use drywall anchors" the shelves might fall off the wall is moronic on its face.

I'm confused now. I just told the cautionary story of our pantry shelving because the OP asked about using drywall anchors in plaster.

Can you post pictures of the back of your shelves?

Queen, no worries. Most of us understood your point. :-)