That happens with almost any local rule, not just this specific illegal one about bunkers
Staked Trees, Preferred Lies, Bunkers GUR all make the course easier but are allowed.
Could you make a local rule that says bunker faces are compulsory GUR? Or can you only do that for entire bunkers?
If the course is made significantly easier by the use of a local rule the course rating should be adjusted to take this into consideration. From what I have seen over the years though , most clubs do not bother. This can definitely lead to players getting handicaps that they cannot play to once conditions return to normal. For instance A club committee makes a local rule " Preferred lie , through the green. which is not an acceptable local rule. This is left in place for 12 months ( it does happen and quite a few members were not happy about it. ) after which the committee changed. The new committee cancels the local rule & suddenly the members,most of them retirees, can no longer play to their handicaps..
Another instance I can think of, a course where all the bunkers have been GUR for over 2 years but no adjustment has been made to the course rating.Most of their members struggle to compete when they play at other courses because their handicaps do not reflect their ability.
Member of the Vision Test Pilot Project.