I thought that it should, but didn't for two reasons:
1) On the original issue where this issue was introduced (
SOLR-7844), writing the leader props to both "shard1" and "shard1/leader" was done in 5x only, suggesting that there was no intention for 6.0 to be backward compatible with 5.x in this regard.
2) Mark Miller made this comment "Yeah, 5x needs a little bridge back compat that checks the old location if the new one does not exist." which again, made me think that there was no intention to support such a use case.
Basically I agree with you, if someone will upgrade from 5.3 straight to 6.0, and will want to do this as a rolling upgrade, then he'll hit that issue. But I wasn't sure if that scenario is intended to be supported. I'd be happy to port this fix to trunk as well if people think otherwise.