Large cloud providers are actually exactly the ones who argued that leap seconds should be abolished, so I presume there were enough use cases where this was an issue. I faintly remember one article about the announcement of the voting results where they talked about the issues that smearing could cause but I don't remember exactly what it was, or where I read it.

Voting running after midnight local time (where leap seconds get inserted) and dependent on sub-seconds, with only five leap seconds since 2000 introduced reads like a really interesting case.

I find it difficult to believe there are use cases affected by this; given that NTP normally has an error window of some milliseconds, those use cases would never have worked.

Wrong. The smearing cumulates during a day...