There’s md (of different flavours; which wasn’t even needed because the conventions md is directly copied from were already sufficient - and yes, let’s argue about it!), then there’s text bundle and there are more, propriety apps have their own plaintext.
So while there actually a plaintext - the plaintext - nobody does the plaintext.
Whenever I need plaintext I just do plaintext using “-“ and tabs+- for lists etc. In case you missed almost everything is there in plaintext - line breaks, paragraph, punctuations, special characters etc. And just too many plaintext apps as well to keep you happy.