Fair point, M

Maybe it's Law 4 - basic equipment, which doesn't refer to anything protruding:

Basic equipment
The basic compulsory equipment of a player comprises the following separate
items:
• a jersey or shirt with sleeves – if undergarments are worn, the colour of the
sleeve must be the same main colour as the sleeve of the jersey or shirt
• shorts – if undershorts or tights are worn, they must be of the same main
colour as the shorts
• stockings – if tape or similar material is applied externally it must be the
same colour as that part of the stocking it is applied to
• shinguards
• footwear

Law 4 was amended to include undershorts in around March 2011 - so why they've only started checking now in the Premier League, I don't know.

Story about a youth team having to change its kit here:Undershorts

LFF