I know in more formal writing, spelling out fractions is preferred (e.g. two-thirds), and in math no suffix is used, but I frequently see ordinal suffixes being used on fractions (e.g. 2/3rds), even from reputable news sources.
2/3 is already pronounced two-thirds and, to me, the addition of rds just looks weird and is certainly unnecessary.
Google turns up millions of examples:
Even legit news sources:
Wikipedia has nothing to say about it:
Is this proper English or just an undefined grey area?