備忘録的に。

Metal の存在有無は，コマンドラインから system_profiler SPDisplaysDataType を実行して，実行結果に Metal: Supported という文字列があれば，Metal がサポートされているということだろうと思います。

iMac Pro (2017) での実行結果:

$ system_profiler SPDisplaysDataType
Graphics/Displays:

    Radeon Pro Vega 64:

      Chipset Model: Radeon Pro Vega 64
      Type: GPU
      Bus: PCIe
      PCIe Lane Width: x16
      VRAM (Total): 16 GB
      Vendor: AMD (0x1002)
      Device ID: 0x6860
      Revision ID: 0x0000
      ROM Revision: 113-D0500D-114
      VBIOS Version: 113-D05001A1XT-018
      Option ROM Version: 113-D05001A1XT-018
      EFI Driver Version: 01.01.114
      Metal: Supported, feature set macOS GPUFamily2 v1
      Displays:
        iMac:
          Display Type: Built-In Retina LCD
          Resolution: 5120 x 2880 Retina
          Framebuffer Depth: 30-Bit Color (ARGB2101010)
          Main Display: Yes
          Mirror: Off
          Online: Yes
          Rotation: Supported
          Automatically Adjust Brightness: No

追記: Big Sur からMetal Family: Supported という感じになりました。

$ system_profiler SPDisplaysDataType
Graphics/Displays:

    Radeon Pro Vega 64:

      Chipset Model: Radeon Pro Vega 64
      Type: GPU
      Bus: PCIe
      PCIe Lane Width: x16
      VRAM (Total): 16 GB
      Vendor: AMD (0x1002)
      Device ID: 0x6860
      Revision ID: 0x0000
      ROM Revision: 113-D0500D-114
      VBIOS Version: 113-D05001A1XT-018
      Option ROM Version: 113-D05001A1XT-018
      EFI Driver Version: 01.01.114
      Metal Family: Supported, Metal GPUFamily macOS 2
      Displays:
        iMac:
          Display Type: Built-In Retina LCD
          Resolution: 5120 x 2880 Retina
          Framebuffer Depth: 30-Bit Color (ARGB2101010)
          Main Display: Yes
          Mirror: Off
          Online: Yes
          Automatically Adjust Brightness: Yes
          Connection Type: Internal

ちなみにM1で実行すると次のようになります。

$ system_profiler SPDisplaysDataType

Graphics/Displays:

    Apple M1:

      Chipset Model: Apple M1
      Type: GPU
      Bus: Built-In
      Total Number of Cores: 8
      Vendor: Apple (0x106b)
      Metal Family: Supported, Metal GPUFamily Apple 7
      Displays:
        PL2875UH:
          Resolution: 3008 x 1692
          UI Looks like: 1504 x 846 @ 60.00Hz
          Main Display: Yes
          Mirror: Off
          Online: Yes
          Rotation: Supported
          Automatically Adjust Brightness: Yes