Hallo everyone,
Warmbacher it's good to see you here ! I'll take your point and I might gradually change admin_level=10 which results of merger municipalities back to admin_level=8 with boundary=historic and the date of merging. That make the most sense. It does not matter whether the old boundaries show up on mapnik or not, but what does matter is that makes the former municipalities still searchable, so it's definitely a better idea (in case someone who was not aware of the merging searches them, they find them).
I've seen people use boundary:disused=administrative or something like that. I do not think it's a good idea, "disused" refers more to physical things such as railways, rather than logical objects such as boundaries (according to my own understanding of the wiki).
Simon, I do not understand what you do mean when you think old boundaries "will eventually decay as they won't be maintained". There is no maintaining to do, since the border just is there and never moves ? Unless you're talking about VERY long term (thausands of years) when earthquakes and such will radically alter the landscape, but at this point I don't think computers will still be around, let alone OSM ! And of course old boundaries can be verified by looking at old maps which are past their copyright date. In many cases boundary stones are there too, although no guarantee.
Mit freundlischen Grüssen,
Meilleures salutations,