It doesn't fully prove absence, because nothing stops you from reposting the same thing with a different block hash. Though people would find you annoying :-) 

 True, but single use seals might fix that.  

 True, but single use seals might fix that.  nostr.fmt.wiz.biz 

 "Hey, you said politically incorrect thing!"

"But back then it was normal"

"No you said it recently, you're cancelled!" 

 An interesting use case, but I doubt anyone would put themselves in such a position! 

 I recently caught nostr:nprofile1qqsdjyv3uv8qq3ztjskqaqk263ctx2h3w9mycgn4hmstmxfh0m75qagpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhsz8rhwden5te0wfjkccte9e5xjemgd35kw6r5v4ezucm0d5hszxmhwden5te0wfjkccte9ehx7um5wfhkx6m9wshx7un89u7vcg6t red handed in a time stamp scam. 

He printed a t-shirt with only a block height of Nostriga, and when I told him that's not a proof, he admitted that in fact he printed the shirt weeks before and just picked an estimate of which block number will be produced during the conference. 

He could not have included the actual block hash of the event before it was mined, because nobody knows the hash in advance.

If he wants to prove that the shirt was produced during the conference, add the time stamp of the block at the beginning of the event on the shirt, and add a picture of him wearing it in an open timestamp proof on the blockchain. 

 Hahaha, legit