30-second answer

PowerShell
$web  = Get-PnPFeature -Scope Web  | Where-Object { $_.DefinitionId -eq '94c94ca6-b32f-4da9-a9e3-1f3d343d7ecb' }
$site = Get-PnPFeature -Scope Site | Where-Object { $_.DefinitionId -eq 'f6924d36-2fa8-4f0b-b16d-06b7250180fa' }
if ($web -or $site) { "Classic publishing is enabled" } else { "Not enabled" }

Two features, two scopes: PublishingSite (F6924D36-2FA8-4F0B-B16D-06B7250180FA) at site scope, PublishingWeb (94C94CA6-B32F-4DA9-A9E3-1F3D343D7ECB) at web scope.

What this proves

Whether the classic publishing infrastructure is activated on this site or web. Activation is a fact the product records as a feature id in the enabled-features list.

What it does not prove

  • Whether anything uses it. An enabled feature is a capability, not usage. Sites carry publishing enabled for years after the last publishing page was retired.
  • What turning it off would break. Deactivation has consequences the feature list does not describe; page layouts and master page settings depend on it.

PowerShell

PowerShell
Get-PnPFeature -Scope Web |
  Select-Object DisplayName, DefinitionId |
  Sort-Object DisplayName

Collect the full id list once and let the interpretation live elsewhere: which GUID means what is a documented claim, and it changes on documentation's schedule, not the collector's.

Example output

Text
DisplayName          DefinitionId
-----------          ------------
MDSFeature           87294c72-f260-42f3-a41b-981a2ffce37a
PublishingWeb        94c94ca6-b32f-4da9-a9e3-1f3d343d7ecb

Explanation

Publishing marks a site as classic-built in the way that most resists modernisation: it brings page layouts, master pages and an editing model that modern pages do not have. Its presence is the single most reliable signal that a site was built as a classic publishing portal, and Microsoft documents it as unsupported territory for the modern experience.

Production considerations

  • Get-PnPFeature -Scope Site and -Scope Web are two different lists; checking only one misses half the answer.
  • Site read access is enough.
  • Compare ids case-insensitively; the product is not consistent about casing across APIs.
  • SPO-MODERN-001: the classic publishing infrastructure is enabled on this site

References

Found something wrong? Suggest a correction. The article source is not public; the engine it cites is.

What this answer underwrites

The engine reads these Microsoft operations to collect evidence, so what is established here is what those collectors rest on.

  • customizationthe surfaces by which executable content or customization can reach a page on one site
  • modernityhow one site is built: template, branding, publishing