I recently had a need to convert a big list of permanent Twitter user ids to usernames. Here's how I did it.
Getting the username from an id is as easy as following what this URL redirects to: https://twitter.com/intent/user?user_id=<Twitter_id>. But doing this is bulk requires using the Twitter API in some way. Registering for Twitter developer account requires verifying your phone number and I didn't want to deal with credentials for this temporary task.
Meanwhile, I found tweeterid.com. It allows you to convert Twitter ids to screen names but you have to do it one by one.
Inspecting the network tab in the developer console, I found that it makes a POST request to https://tweeterid.com/ajax.php with the data input=<twitter_id>. So for my list of ~400 ids, I made a file named twids with each line containing one id, and ran the following command in my terminal:
xargs -I {} curl -s -d 'input={}' 'https://tweeterid.com/ajax.php' -w '\n' < twids
This command takes each line of twids file, makes a POST request with the data, and writes out the response to stdout separated by newline characters.