Written as of PowerShell Core 6.2.0
The reason is that PowerShell treats the following:
${<drive>:<name>}
as if you had specified:
Get-Content -Path <drive>:<name> # or, with assignment, Set-Content -Path ...
This notation - though often used with the Env: drive (e.g., $env:Path) - is little-known as a general paradigm named namespace variable notation, which is explained in this answer.
The problem is the use of -Path rather than -LiteralPath, because -Path interprets its argument as a wildcard expression.
Therefore, the [foo] in ${env:[foo]} - rather than being used as-is - is interpreted as a wildcard expression that matches a single character that is either f or o ([foo] is a character set or range ([...]) that matches any one of the (distinct) characters inside - see about_Wildcards).
On assigning to ${env:[foo]}, the logic of Set-Content -Path requires that a wildcard-based path resolve to something existing, even though you're generally not required to explicitly create environment variables; e.g., ${env:NoSuchVarExistsYet} = 'new' works just fine.
Workaround:
Use double(!)-`-escaping of the wildcard metacharacters:
# Namespace variable notation only works with if you
# double(!)-backtick-escape the wildcard metacharacters:
# Assign to / implicitly create env. var '[foo]'
${env:``[foo``]} = 'bar'
# Get its value.
${env:``[foo``]}
Note:
Escaping shouldn't be required at all, because there is no good reason to treat paths that conceptually identify a given, known item as wildcard expressions - see GitHub issue #9225.
That double `-escaping is needed is an added quirk - see GitHub issue #7999.
Another workaround - one that doesn't involve escaping - is to use
Set-Content -LiteralPath env:[foo] bar and Get-Content -LiteralPath env:[foo], but that is both verbose and slow.
As for the other syntax variations you tried:
$env:${[foo]}="bar"
Since your variable reference isn't {...}-enclosed as a whole (except for the initial $), the token that follows the : is only allowed to contain characters that do not require escaping - and $, { and } all violate that rule.
{...}-enclosing the entire path - ${env:[foo]} - solves the syntax problem, but runs into the problem detailed above.
Set-Item -LiteralPath env:${[foo]} -Value "bar"
This does not work in general, because string expansion is applied beforehand here - it is as if you had passed "env:${[foo]}": the reference to a (regular) variable named ${[foo]} is expanded (replaced with its value) and in effect appended to literal env:, before handing the result to Set-Item.
If such a regular variable doesn't exist, what Set-Item sees is just env: (because non-existent variables default to $null, which becomes the empty string in a string context), which causes an error due to the lack of variable name.
By contrast, the following would set an environment variable named unrelated instead:
# Create a regular variable literally named '[foo]'.
${[foo]} = 'unrelated'
# !! The following sets env:unrelated, i.e., env. var 'unrelated',
# !! due to the string expansion that is performed on the -LiteralPath
# !! argument up front.
Set-Item -LiteralPath env:${[foo]} bar
$env:unrelated # -> 'bar'
The same applies to Get-Item -LiteralPath env:${[foo]} and
Set-Item -LiteralPath env:${[foo]2} -Value "bar".
