I'm pretty sure this is a breaking change.

Ubuntu18.04 seems to default to C.UTF-8 for LANG (representing InvariantCulture)

Could you please reference where it come from and why?

Is there other cultures like C.UTF-8 on Unix and Windows with the problem?

@iSazonov read the details in the related issue

Perhaps it makes sense to consider wider the culture-insensitive and case-insensitive in PowerShell Committee review. Related #8064 (Not all code base fixed as mentioned in #8068).
For #8120 and #8515 we need find all places where we could be the culture-insensitive and case-insensitive and use the simple case folding comparer.
Another side is issues like #8180. Perhaps sometimes it makes sense to have default to ordinal ignore case in string comparisons.

@iSazonov In my opinion this pull request correctly fixes the issue described in #7761 and we should keep wider discussion of changing other culture-insensitive in other places out of this pull request.

As this is a serious issue in a core functionality (Hashtable) and as it is present on the default culture of Ubuntu 18.04 (C.utf8) and thus exposed to a large number of users in my opinion this should be merged with high priority and be backported to 6.1.X and 6.0.X.

My request is not to block the PR. My fear is that there may be other places prone to this problem. This analysis may be useful for fixing other issues.

@PowerShell/powershell-committee reviewed this and agrees to take this change as the intention was to always have Hashtable keys case-insensitive

@iSazonov Only one CL-* tag. If it is a breaking change, don't put it in one of the others.

Wow, thank you for fixing this. I am new to PowerShell Core and using it on Linux, and it seemed that this was intentional. I sent a lot of time fixing the casing of my hash tables. I'm guessing this is also the issue I ran into with sort-object, which gives different results on Linux and Windows:

PS> $PSVersionTable.Platform;('a', 'ab', 'aC' | Sort-Object ) -join ','
Win32NT
a,ab,aC
PS> $PSVersionTable.Platform;('a', 'ab', 'aC' | Sort-Object ) -join ','
Unix
a,aC,ab

@adamedx I think it would be a good idea to track this in a separate issue and link it to #8064 where similar issues are being tracked.