A farewell to Ada with null (1992)
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Tucker Taft has been the lead architect for all post-83 versions of Ada, so there's some consistency in the direction of additions.
If Ada83 is analogous to C, Ada95 would be analogous to C++, with subsequent versions of Ada being analogous to subsequent versions of C++.
I'd personally argue that the situation we're in today, with one company (AdaCore) being responsible for the only open source Ada compiler out there, is due in large part to the large increase in complexity of the Ada language (the Ada83 reference manual was 348 pages while the Ada 2012 reference manual is 951 pages).