To be clear, $EnabledExperimentalFeatures - which apparently isn't documented yet (see MicrosoftDocs/PowerShell-Docs#9722) - should obviously never surface obsolete experimental features, but as for why you're seeing it:

Presumably, your user-level powershell.config.json file ("$(Split-Path $PROFILE.CurrentUserAllHosts)/powershell.config.json") still lists this obsolete feature in its "ExperimentalFeatures" property, and removing it should make the problem go away.

That raises the question whether it's worth issuing a warning when an obsolete or invalid name is encountered (an invalid name is currently quietly ignored, unless its name starts with Microsoft. and is followed by at least one letter), at least in a session invoked with neither -File nor -Command

My powershell.config.json file contains:

{
    "Microsoft.PowerShell:ExecutionPolicy": "Bypass",
    "ExperimentalFeatures": [
        "PSNullConditionalOperators",
        "PSSubsystemPluginModel",
        "PSDesiredStateConfiguration.InvokeDscResource",
        "PSAnsiRendering",
        "PSAnsiProgress",
        "PSAnsiRenderingFileInfo",
        "PSNativePSPathResolution",
        "Microsoft.PowerShell.Utility.PSManageBreakpointsInRunspace",
        "PSNativeCommandArgumentPassing",
        "PSStrictModeAssignment",
        "PSExec",
        "PSCustomTableHeaderLabelDecoration",
        "PSModuleAutoLoadSkipOfflineFiles",
        "PSNativeCommandErrorActionPreference"
    ]
}

So you can see there several features that have gone mainstream and two that have been removed. The question is why does "Microsoft.PowerShell.Utility.PSManageBreakpointsInRunspace" show up when the others don't.

It probably is due to the magic that my previous comment alluded to (I haven't checked the source code): that Microsoft.-prefixed names, wether or not they refer to current or former experimental-feature names, are seemingly always surfaced in $EnabledExperimentalFeatures.

$EnabledExperimentalFeatures is not string[]. That is why wrong comparing:

That Compare-Object doesn't recognize $EnabledExperimentalFeatures as enumerable is an interesting find and worth reporting as a bug in its own right.

However, it is incidental to the problem at hand, whose repro bypasses the issue by piping $EnabledExperimentalFeatures to Sort-Object first.

To close the tangent re $EnabledExperimentalFeatures not acting like an enumerable with Compare-Object: see #18990