One of the Google I/O talks mentioned that this feature (currently behind a feature flag) is probably coming to Canary in a few weeks:
I think it might be particularly useful for priorities between JS files and images. I.e. the article images near the top (eg. first 3 in-article images) could be given a high priority, higher than async JS, and the rest of the images given a low priority. And between JS files, we could also have different priorities.
Chromium discussion: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/65lfM2f0eeM/-1ttGbZmCQAJ
And code review: https://chromium-review.googlesource.com/c/chromium/src/+/988955
Behind a flag in 70+ https://www.chromestatus.com/feature/5273474901737472
Available as an origin trial on 73,74 until May 29: https://developers.chrome.com/origintrials/#/view_trial/945755921747804161