Tradetech

Slugs are crafty creatures they can fit through small gaps and crawl up pipework drainage maybe your cistern lid is not fitted properly small gap gaps leading through your wall or floor check carefully for any small gaps or holes cracks and seal them up this should holt the pesky so and so's but they may come under the door damp dark and cool they like it that way!??

Outside garden taps should have one any tap being used with hose attachments shower attachments really any tap or water outlet that has or will have or potentially have an attachment that can fall below the waterline
outside taps
bath taps
showers with shower hose that is long enough to become submerged
i can see situations where any outlet could have backflow filing a bucket with water in a sink and the fill line is above the clearance of the taps
definitely baths,toilets and showers sinks and basins its a good idea and outside taps i think the plumber just didn't think about it
what happened to cause the issue what tap was it you don't say that would make a difference on weather he was incompetent or just not mindful of potential issues!!! and he may have assumed there was adequate prevention in place again yes he should have checked but not all do!?
and it really depends on how the issue came about if it was an outside tap plumber could be at fault a shower with an obvious backflow risk plumber a bath 50/50 but a kitchen sink thats hard to judge the blame and how the water got high enough to backflow is really the question if it was not an outside tap shower bath or toilet and to cause extensive issues in other flats how long was the water backflowing? i need more info to make a proper assessment