I've noticed players are having a hard time reporting cheaters/glitchers/harrassers who change name and have been requesting a 'UplayID' in order to find players. However, players do have a unique ID but just not as handy as a SteamID. If you already know what I'm talking about that's okay this post isn't for you.
Retrieving an ID
You may have visited the recently updated Rainbow Six Stats website. It's restructuring has made things much easier.
Log into the website using your Uplay login to view your statistics (select your platform top right if necessary). Once here you'll see a string of characters (example: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx) in the address bar at the top between 'player-statistics/' and '/multiplayer'. The string of characters is your unique ID on Uplay.
Stats Website Example
Now if you click on the search field top right, enter the name of another player and view their profile. Once again in the address bar is their ID. No matter what name they change to that ID will let you find them. You can also click 'follow' below their name to keep track of them. I'd recommend keeping that for friends and ESL players.
To look up a player via their ID; add it to this link and copy/paste it into the address bar (change uplay to psn or xbl for other platforms).
https://game-rainbow6.ubi.com/en-us/uplay/player-statistics/[PLAYER-ID]/multiplayer
When you have their ID it can be submitted along with your evidence to Ubisoft Support. This should make it easier for the support staff to find the player you have encountered.
Other ID Locations
Cookie
Another way to get your ID is to check your cookies. Ubisoft.com leaves it under 'user_id'.
Cookie Example
Local PC
The ID can also be found locally on your PC if you navigate to where Siege stores your config file.
"C:Users[PC-USER]DocumentsMy GamesRainbow Six - Siege"
Folder Example
Advice
NB. Don't post player IDs on the subreddit with screenshots or videos of wrongdoing. It will be removed for witchhunt reasons.
NB2. Don't make a public list of supposed cheater IDs because that would be dumb and pointless.