Hello,
First of all, thank you for the good deed developing OSM Boundaries!
I've found this recipe (https://trac.osgeo.org/postgis/wiki/UsersWikiSimplifyWithTopologyExt) to fix "gap" between boundaries due to ST_SimplifyPreserveTopology limitation and wonder if OSM Boundaries will implement this. If it's working as they said, I think it would be a level up in OSM Boundaries' UX.
UPDATE:
this is probably not going to be considered, giving that the owner didn't even bother to reply with a simple "yes" or "no".
so, for everyone needing polygon simplification, I suggest to download the borders without simplification from here and simplify them here https://mapshaper.org/
this is an excellent simplifier that preserves topology, small polygons, corrects intersections and avoids gaps/slithers
We are sorry for not answering this earlier. The fact is that we very much liked this suggestion, but the using the method on that Wiki page results in quite a lot of issues.
We have however found another method and this far we are quite happy with the result. We are expecting/hoping for a release this week.
please add this, it would be lifechanging