<?php

$prodi = [
    "TI-D3TI", "TI-D4RPL",
    "TM-D3TM", "TM-D4PM"
];

$new_prodi = [];

foreach ($prodi as $prod) {
    # prefix = TI, TM
    # postfix = D3TI...
    [$prefix, $postfix] = explode("-", $prod);

    # push postfix
    $new_prodi[$prefix][] = $postfix;
}

var_dump($new_prodi);