ITU 2017 Competition Rules, Transition Area Conduct: "k.) Marking positions in the transition area is forbidden. Marks, towels and objects used for marking purposes will be removed and the athletes will not be notified;"
I looked at the Ironman 2017 Competition rules and there is nothing in there forbidding marking your transition area. But as I noted earlier, a lot of foreign IMs/70.3s don't allow you to mark your bags. Below is from the 28 Jan 2018 Athlete Guide for IM70.3 Buffalo City, South Africa
"â€˘ Transition bags x 3 : Street wear (white), Bike (blue), Run (red). Bags may not be decorated for personal identification purpose
This markings rule is not a new rule, and for sure is enforced at pretty much any sanctioned race in Canada... and certainly at ITU level events... And yes, many people are worse off, because they were relying on something silly like a balloon or the sort for finding their spot upon their return, rather than actually landmarking like the rest of us do...
Also worth noting that most races will not allow a snorkel at all, not just disqualifying you from awards...
as a basic starter article it does a decent job outlining some of the basics to people.