サンプルのクエリ：

?a=0&b=2&a=1&d[]=0&d[]=1&こんにちは=hoge=piyo

値にイコールが含まれてることと配列を考慮しない

const $_GET = Object.fromEntries(decodeURIComponent(location.search).slice(1).split('&').map(prop => prop.split('=')));

結果：

{
  "a": "1",
  "b": "2",
  "d[]": "1",
  "こんにちは": "hoge"
}

配列を考慮しない

const $_GET = Object.fromEntries(location.search.slice(1).split('&').map(prop => {
    const data = prop.split('=');

    return [data[0], decodeURIComponent(data.slice(1).join('='))]
}));

結果：

{
  "a": "1",
  "b": "2",
  "d[]": "1",
  "こんにちは": "hoge=piyo"
}

配列も考慮する

const $_GET = (query => {
	const dataSet = {};

	for (const data of query) {
	    const key = data[0];
	    const val = decodeURIComponent(data.slice(1).join('='));

	    if (!key) {
	        continue;
	    }

	    if (key.endsWith('[]')) {
	        const _key = key.slice(0, -2);

	        dataSet[_key] ??= [];
	        dataSet[_key].push(val);

	        continue;
	    }

		dataSet[key] = val;
	}

	return dataSet;
})(location.search.slice(1).split('&').map(prop => prop.split('=')));

結果：

{
  "a": "1",
  "b": "2",
  "d": [
    "0",
    "1"
  ],
  "こんにちは": "hoge=piyo"
}