i guess i might get batting for this but will ask since no clue in the field, say OP has customers that wants t shirt with that logo leaving aside russia 2018, and if he pulls if off in single color the shape is really nothing new, as long as he doesnt claim its his own design and doesnt go wholesale wheres the damage ?
ex: i own mazda, so made myself mazda 3 logo on doors ,made few for friends that own bmw's audi's, but could easily type into google search and it will pull up 100s of sites that do em, and guarantee none of em have any relations or business contracts with actual dealerships nor companies to use it, in other words where is the line drawn, as ive taken some designs of net , would trace them myself and usually make modifications - always room to improve on many and make em look better be it colors shapes or shading/style.
I get general idea of knock off's say you decide to use logo of a brand on item to push it as real deal, but what about individual designs you take one and improve it and use for personal use, i guess theres some way to find sometimes original creator but many times you get 100s simillar so might as well end up paying for nothing to someone.I know USA might be more strict but speaking globally how does one break law if design is close enough but isnt actually original, you can go buy mona lisa copy online and you still end up with close enough copy and who will nail you for that.
Since my personal experience is if its not your made design theres 0 proud to be off, but dont see harm if someone puts it on t shirt as it will be like one between millions if were talking huge brands or logos.