SSDs and 64K sectors SSDs tend to have a natural 4K sector size as far as I know, but would it be beneficial to format Windows cluster size at 64K? This is good practice for spinning rust, but is it relevant on SSDs?
Yes, 64KB is still the current recommendation as it has to do with partition alignment.  To quote Jimmy May from the the SQL CAT team:


  Partition alignment remains a best practice for all versions of
  Windows Server as well as SQL Server, including SQL Server 2012 &amp; SQL
  Server 2014.  No exceptions.  Period.


As part of the Best Practice guide referenced in that article, the following line is quite explicit:


  The file allocation unit size (cluster size) recommended for SQL
  Server is 64 KB


As for a SSD vendor's perspective, I point you to Argenis Fernandez's blog post about this very topic.  Pure even recommends 64KB for NTFS block size, so I think it's safe to assume this is a pretty universal truth.