6-speed bikes and 7/8/9/11 speed chains
The Wikipedia section on chain widths glosses over some history/is wrong.
Pre-indexing derailleur chains, i.e. what would have come on a 6-speed bike in 1980 and what many would call or label as a 6 or 5/6 speed chain, have a pin length of at least 7.6mm and usually around 8mm. New chains of this type are still produced and available but are not stocked very commonly in many places in the world. The ultra low end repair market is a bigger deal in some places than others. They can be replaced with 8-speed chains (most of which currently produced having a 7.3mm pin length) at essentially no detriment to performance. Here is a picture of one sold under J&amp;B Importers (one of the largest US bike parts wholesalers) house brand:

Contemporary 6-speed freewheels for indexed shifting have ramping etc and are mostly designed to be used with 7.1-7.3mm (aka 8-speed chains). I don't actually know much about what happens when you put the above 1970s style chain on them because I almost never touch those. I believe it works to drive the bike as the spacing is still right, just the shifting is bad or suboptimal with an indexed shifter.
The &quot;going a speed generation narrower is fine&quot; line for more modern index 8-12 speed bikes is bad, overly broad advice that's parroted much too frequently. Doing so causes laggy shifting in back and the mismatch with front derailleur cage inner width usually causes compromised performance in front. There are some various situations people get in that lead them to do this, a big one being putting an n+1 speed crank on an n speed bike and finding that the n speed chain rubs on the big ring in the inner/middle-small combinations, but there's almost always a better solution. In my experience, the issues with this are a little less pronounced with 9-speed-chain-on-8-speed-bike than in the latter generation mismatches, and that's where the notion may have come from.