So in a word, timestamping right? Because if you want to have non-renegable contracts then any digital signature will do, and you can sign a commonly agreed tim

So in a word, timestamping right? Because if you want to have non-renegable contracts then any digital signature will do, and you can sign a commonly agreed timestamp into it like "I will provide service X as of 2020-10-24T21:57:14+0200 <insert signature>". The only thing you can't prove is when it really was signed, sent, published, that sort of thing, and for that a timestamping service can be used (which other comments already mentioned).