In 2016, FIFA fined the home nations for displaying poppies at international football matches. Players cannot wear political or religious images, either on their shirts or on armbands. More recently, they have suggested that poppies may be allowed. They've now tightened up their definition of a 'political symbol' - though it is still broad enough to include any national government and also any specific political act or event (which seems circular).
What seems strange, then, is that they never seem to have had a problem with national flags or emblems such as the 'three lions' on England shirts (which are the royal arms of the Plantagenet kings). Several other nations, such as Italy and Norway, wear shirts featuring their national flag. Indeed, in some cases even the colour of the shirt can signify political and/or religious allegiances, hence the strong feelings over green and blue around certain parts of Glasgow. Surely, if all political and religious symbols are forbidden, these are also problematic...
I can sympathise with those who wish to keep politics out of sport, but apparently it's impossible to keep sport out of politics, as shown by President Trump's recent comments over NFL players and the national anthem. Plenty of people seem to be boycotting the NFL because they don't want politics in their sport, but in that case why is there a national anthem in the first place? And is anyone boycotting Trump for putting sport in politics?