I just made a little tool which might help server administrators to track their user's namechanges.
The download is a .zip-file, which contains 2 .jar-files, one for BungeeCord and one for Spigot (it's called Bukkit actually, but works for Spigot too).
After dropping the correct .jar-file into the plugins-folder (if you use BungeeCord, you only need the BungeeCord-.jar-file, not both), just restart your server.
By entering /namelookup PLAYERNAME, you send a request to mojang.com to get the players' uuid and all usernames the account had. This will run asynchronously, so it won't freeze your server. Then you will get a list all usernames the player had:
If you have the BungeeCord-version, you can click on the uuid or one of the names, the uuid/the name will get pasted into your chat-input field, so you can copy it easy from there.
This plugin only uses the mojang.net-api, no third-party supplier-api!
I hope someone can use this plugin, I'm open for new feature-requests.