Over here, a lease implies that it's RIPE NCC's job to make sure that the prefix is and remains in usable shape for its intended purpose (globally routeable, not blacklisted etc.).
does RIPE NCC -really- provide assurance that a prefix it hands out is globally routable?
At least for PI space, they explicitly say they don't. (Of course, this doesn't completely rule out that an implied guarantee exists nevertheless.)
` Or is that just an implied aspect of the term "lease", which is not in use these days?
Yes, it's a mandatory part of the contract, the leaser cannot abstain from that. You've got some wiggle room with regard to the scope of the contract. But a lease of something that provides no documented value (and the contract explicitly saying so!) will be a very difficult sell to various parties. It borders on breach of trust ("Untreue", embezzlement in extreme cases). The current model of industry self-governance does not seem to suffer from that.