I know of a few of highly custom enterprise systems. I still do a fair amount of consulting hours on 2.7 codebases. There is no technical reason why they couldn

I know of a few of highly custom enterprise systems. I still do a fair amount of consulting hours on 2.7 codebases. There is no technical reason why they couldn't be migrated, it just economics (or vendor management with incentives to not make it happen). The systems get to age out and are replaced with something else or rewritten from scratch.