You are right, holding giveaways for Robux or other items via things like Twitter, Instagram ect; is against TOU (TOS) and I have seen a few people get their account banned for it but it largely under the radar as Twitter is a 3rd-party website/app so they cant just have people skimming threw every day, tracking down all the giveaways then finding the user that started them. It would just cost too much for them!
So my advice would be to try and stay away from hosting giveaways on things like Twitter as its not ‘ok’
Third parties sell or provide Game eCards (which are physical cards or digital items that provide a code that you can use to redeem Robux or other virtual items on Roblox). Those Game eCards have their own terms and conditions associated with them. If you want to sell, give away or do anything else with those eCards outside of Roblox (on an external website or social media channel), then we take no position as to those actions. However, we recommend you check the terms and conditions of the eCard and place you want to do the action to see if that is allowed.