There are some texts left by an ancient civilization in India. They were written around 2700-1800 BCE. They have not been able to decipher them yet. Is it possible that the texts were Indo-European? Or, could some of the later texts be Indo European because they wasn’t contact yet? Just curious. If they are, it probably beats Linear A if it is. If it is, the language may be the closest thing to Proto-Indo-European there is that has been recorded. Note: religious items may show what they believed, and similarities with other IE religions, including Hinduism, could be used as evidence.
It is not even established that those symbols represent a language, because statistically they do not "fit" within either syllabary, alphabet or logographic systems. If we aren't even sure they're language, we cannot possibly guess what family language they'd be, although the IVC is sometimes posited to have been migrated south forming the Dravidian cultures, in which case they wouldn't be IE.
It is highly dubious that the Indus Signs might encode an Indo-European language, because, according to all probabilities, no Indo-European language was present in that area at the time this system was used.
Attempts at "reading" Indo-Aryan in the Indus Signs are mostly motivated by political (nauseating...) agendas and they do not resist basic counter-check. The received scientific opinion is that speakers of Indo-Iranian languages arrived in the Indus Valley area, after 1500 BCE, that is to say after this civilisation had already entered a process of decay and collapse.
The claim that the Indus Signs cannot encode language is doubtless false. The statistical profile of the Signs is consistent with a syllabary of the Japanese type, that is to say a syllabary with quite a lot of signs (< 150). Besides, it is possible that the system is a mix of phonetic symbols and ideograms, which makes deciphering even more difficult.
Some seals clearly seem to be of accountancy nature, with signs and numbers. Even Sproat who wrote papers with Farmer and Witzel is ready to concede that some seals do look like accountancy.
The main problem is that inscriptions are all short and we have no idea what the language(s) is/are. A similar issue exists with LinearA in Crete.
The nature of the corpus makes deciphering difficult, and at the same time, it is hardly possible to prove that an attempt at deciphering is correct.