Bear in mind that the previous state of record updates for GADTs was complex, buggy and ill-specified. The new design is much clearer: simply apply the translation of record updates in the Report.
GHC frequently fixes bugs with the side effect that code that was (accidentally) relying on the bug ceases to work. Volunteer contributors aren’t going to be pleased if every patch that fixes a bug also has to introduce a warning, retain/test the old code path for two versions, etc. and it seems like it would lead to a significant increase in the internal code complexity. Basically I don’t think it is feasible to have GHC maintain bug-for-bug compatibility between versions without a very substantial increase in the resources going in to maintenance.