remove
curl_setopt($UST, CURLOPT_POST,true);
You ain’t posting you are PUT, so the curl_setopt($UST, CURLOPT_POST,true); is Not Applicable and may interfere, the CUSTOMREQUEST overrides and the POST may set other stuff, you only need the CUSTOMNREQUEST
As to the actual probblem:
Your body that you are sending is wrong I believe (as you stated with your var_dump output), since you need to send an object containing a key of tag_ids that is an array of ID’s
The body of this request should be:
$USTarray = [
'tag_ids': [
"80427d95-bb46-42d3-bf4d-408e9bdca49a",
"96b6073f-450d-4248-8ed4-988e28f3f759",
"cc8d5abb-39c9-4942-a1ee-e1558512119e",
"7616f6ea-7e3d-4501-a87c-c160d2bc1849",
"26301bb2-91a3-4272-8a9b-5bcea6db0fcd"
]
];
$USTinput = json_encode($USTarray)
This then matches the example on Reference | Twitch Developers