You didn't say which Mac you have. However, even on my ancient High-Sierra-unsupported MacBook5,1 Core 2 Duo 2.0 GHz with High Sierra, I can get iPhone 4K HEVC playback through QuickTime that is stuttery but usable enough to get the gist of what the video is about. But in Photos, I get that stupid message.
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This is bad form on Apple's part. No need for this, assuming it's not a bug. Also, even if they did have a good reason to throw up an error message, this particular error message is very confusing, because the format actually is fully supported even on old machines. In fact, Apple itself says it's fully supported:
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It's just that some machines are too slow to play it back smoothly. However, here is Apple's more recent support page on this:
https://support.apple.com/en-us/HT207022
Working with this media
Support for HEIF and HEVC is built into iOS 11 and macOS High Sierra, letting you work with this media in a variety of apps, including Photos and QuickTime Player.
On some older devices, support for HEVC is also affected by the resolution and frame rate (fps) of the video:
- iPhone 6, iPhone 6 Plus, and iPad Air 2 work with HEVC video that was captured at no greater than 1080p/240 fps.
- iPhone 5s, iPad Air, and iPad mini (Retina models) work with HEVC video that was captured at no greater than 1080p/60 fps or 720p/240 fps.
- Mac models from mid 2015 or earlier work with HEVC video that was captured at no greater than 1080p/240 fps.
If you're using iCloud Photo Library with iOS 10 or macOS Sierra, you might see in the upper-right corner of the photo or video, or you might see an alert message. To fully view, edit, or duplicate HEIF and HEVC media on your device, upgrade to iOS 11 or macOS High Sierra.