Since we've removed this check, is it possible that this rule will now flag errors for SASS/CSS-in-JS users? If so, I imagine that this might be slightly breaking for those users (and/or something we propagate to configs)?

(I could also be missing something about how SASS and CSS-in-JS play together with this rule, or with @csstools)

Good catch :)

module.exports = function isStandardSyntaxFunction(node) {
	// Function nodes without names are things in parentheses like Sass lists
	if (!node.value) {
		return false;
	}

	if (node.value.startsWith('#{')) {
		return false;
	}

	// CSS-in-JS interpolation
	if (node.value.startsWith('${')) {
		return false;
	}

	// CSS-in-JS syntax
	if (node.value.startsWith('`')) {
		return false;
	}

	return true;
};

None of those cases would be parsed as a FunctionNode by @csstools/css-parser-algorithms.

Only <ident><open-parenthesis> (e.g. foo() starts a function in standard CSS.

This behavior makes isFunctionNode the equivalent of isStandardSyntaxFunction.

is it possible that this rule will now flag errors for SASS/CSS-in-JS users?

no