The rules of soccer are published in a document called The Laws of the Game. According to the Laws of the Game (2018/2019), Law 04, Section 5:
"Equipment must not have any political, religious or personal slogans, statements or images...For any offence the player and/or the team will be sanctioned by the competition organiser, national football association or by FIFA.”
"Whilst ‘religious’ and ‘personal’ are relatively easily defined,‘political’ is less clear but slogans, statements or images related to the following are not permitted:
any person(s), living or dead
any local, regional, national or international political party/organisation/group, etc.
any local, regional or national government or any of its departments, offices or functions
any organisation which is discriminatory
any organisation who [whose] aims / actions are likely to offend a notable number of people
any specific political act / event
"When commemorating a significant national or international event, the sensibilities of the opposing team (including its supporters) and the general public should be carefully considered."
For a detailed account of how placing the LGBT rainbow on the kit violates almost every aspect of the Laws' definition of "political," see pages 4-10 in the 2019 report, "Let All Play: Yes to Soccer, No to Politics." CLICK to read the report.