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To be honest nothing has changed since Lucien Juy / Simplex design, nowadays derailleurs are still a collapsible parallelogram. The lonely other improvement was indexing.
From Simplex 1975 catalogue:
https://www.velo-pages.com/main.php?g2_itemId=20151&g2_page=2
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The lonely other improvement was indexing
That's not strictly true, Suntour's decision to slant the parallelogram and Shimano's use of a floating sprung B-pivot could both be called material improvements at the time, although they're not considered essential as SRAM (current owners of the old Sachs cycling properties) is now back to slantless parallelograms and fixed B-pivots on their X-Horizon range of 1× RDs. Plus ça change...
How confident are we that this is the right conclusion? It seems to me that it's likely that both Shimano and Sachs were independently inspired by the improved Lucien Juy design then being offered by Simplex, e.g. SLJ6000 from 1975