The phrase "steadfast rule" refers to a firm or unwavering guideline or principle that is followed consistently. It implies that the rule remains unchanged and is not easily influenced or affected by external factors. Full definition
The above recommendations are not steadfast rules, but guidelines to follow. (healthline.com)
Some organizations have some very steadfast rules about how high the kettlebell should end up at the apex of the Swing exercise (primarily they say it should be at a completely parallel position to the ground), we only have guidelines instead. (onnit.com)
These five handy tips are by no means steadfast rules and might not work for everyone — after all, your outfit should showcase your personality. (eharmony.co.uk)