The formal notion of subtype seems to be getting some solid research attention; see e.g. https://drops.dagstuhl.de/opus/volltexte/2021/13888/pdf/LIPIcs-TYPES-20

The formal notion of subtype seems to be getting some solid research attention; see e.g. https://drops.dagstuhl.de/opus/volltexte/2021/13888/pdf/LIPIcs-TYPES-2020-9.pdf https://drops.dagstuhl.de/opus/volltexte/2021/13888/pdf/LIPI... Hopefully we see these ideas make their way into functional languages soon.