Based on a suggestion by @AikenBM, welcomed by @SteveL-MSFT:

Similar to how @(...) ensures treatment of any value as an array, it would be handy if an -AsArray switch could instruct ConvertTo-Json to treat its input as an [ultimately JSON] array, so you can guarantee that your pipeline input is treated as an array, even if it happens to be a single item only:

Wishful thinking:

> $in = 1; $in | ConvertTo-Json -AsArray
[ 
  1 
] 
# scalar 1 was treated as an array

> $in = 1, 2; $in | ConvertTo-Json -AsArray
[ 
  1, 
  2 
]  
# Input that already is an array is left as-is

Awkward workaround, in the absence of -AsArray:

> $in = 1; , @($in) | ConvertTo-Json -AsArray
[ 
  1 
] 

Environment data

Written as of PowerShell Core v6.0.1