Using block-scoped declare const and declare let in global context has a potentially unwanted effect - they can be accessed using unqualified access, but not using globalThis. When this behavior have been implemented in TypeScript (microsoft/TypeScript#30510) all declarations in standard lib files have been changed to declare var, so I believe it's something that generally can be recommended for external declarations too.

// Invalid
declare const foo: string;

// Invalid
export {}
declare global {
	const foo: string;
}

// Valid
export {}
declare const foo: string;