This is a question for custom hardwood furniture builders.
We have an old "secretary" that would have been made in the 1800's. The shelves in the upper bookcase section each sit on two wooden cross members which in turn are inserted unattached onto wooden pilaster strips that are attached vertically in each corner of the cabinet. I can only describe the pilaster strips as being cut in a sawtooth or zig-zag pattern with the bottom of each tooth being horizontal. The system is very simple but robust and works well with shelf height adjustment being in roughly one inch increments.
Are there any custom/premium furniture and/or cabinet builders here who know if such a system is still used when the usual aluminum or plated, mass produced slotted type are not wanted?