HTML Imports are deprecated in favor of ES6 modules and are only supported by blink-based browsers.  The common pattern for usage is to feature-detect support for imports and if it is not supported, use a Javascript polyfill.

Polymer 1.x is the main user of imports and degrades gracefully to the polyfill when not present (polymer 2+ uses ES6 modules): https://www.polymer-project.org/1.0/docs/browsers

The current use counter for HTMLImports shows ~3% of PageVisits triggered an import but that doesn't account for sites that would gracefully degrade with a polyfill.

Recommended path:
1 - Scan the HTTP Archive for sites using imports and verify they all use feature detection and a polyfill fallback.
2 - Send an "intent to unship" with the plan for feedback.
3 - Remove "import" from the link element IDL so feature detection will fail (while actual imports can still be used)
4 - Revisit the HTMLImports usage counter and make sure it is near-zero
5 - Remove support for imports entirely.