Linus has somewhat recently clarified that position regarding old stuff; they break userspace if the break has happened years ago and nobody said anything in the meantime about it. If you notice upgrading from 2.5 to 4.20 breaks your app, nobody at the LKML or any subsystem maintainer is likely to fix it.

A.out is so positively ancient that it's likely been broken for years, nobody is testing or using it in live systems anymore.