People have been arrested & convicted of religiously aggravated offences , nil by mouth say it’s Sectarian and even Paul Mcbride said so.
I understand some feel it isn’t , I understand some feel Fenian isn’t . In society it’s often best to let either those in a position to make the decision legally and dispassionately or actually even those the subject of the slur to decide if it’s offensive as we’ve done with many racist comments or actions in the past .
I don’t know the answer to be honest with you but given the religious and political connotations associated with the old firm I’m unsure it is the same as calling them the RS. Let’s be honest I’ve no real idea of the religious affiliations of anyone I go the game with unless I’ve been to a wedding or christening and the same with RS I know so it just doesn’t matter . To many , not all I’m sure , in Scotland it does.