They are a "bannable" offence, but I've never seen it stop those that wanted to do it. Over the years I reported so, so many, and to this day almost all of those accounts still exist. Guilds full of ABC001, ABC002, ABC003, etc are still easy to find if you look. Alts have been so inconsequential for a long time now I'm not sure why anyone would go to the trouble to report one anymore if it wasn't messing with top guilds.
I think they'd still be a little consequential if they weren't intervened on. Not the same low-effort ways as in the old days when you'd just go "24 more FP generated per day! free FP!", but if you developed the city you could start using it to "push" in various ways.
i.e. twice the space for event buildings to funnel the benefits into your main, self-filled GB spots, "free" sacrificial attrition on tough sectors. But if you got into that high-effort territory I imagine you're more likely to attract a ban than a placeholder-alt to keep "your" guild in place. And they have their auto-system to restrict people on the same connection from donating to each others GBs which covers most of the cases.
Possibly more to the point, because multiple players from the same household are a thing - As long as that other account *isn't* pushing your main, I think it's typical to say it's your roommate/brother/wife/whatever's city, which is technically fine. Even though it might be pushing the boundaries of truth, it's something they can't prove. So as long as it doesn't cross the line into the above territory of "no really, it's a push account, even if it is someone else doing the pushing", I think it'd escape retribution.