Thanks for reply @amandus. It looks like that bridleway issue is the same one I have spotted. All bridleways are accessible by foot in the UK/Channel Islands. I’ve just checked the history and it was I who added it (as a cycle path) 12 years ago! (I did a lot of Guernsey back then - a really enjoyable time going along miles and miles of footpaths and roads I’d never been on). It was changed to a bridleway about 3 years ago and that is where the cycle/foot access must have flipped to ‘Not Specified’. I suspect there might be a lot of bridleways that don’t have foot access and the field is blank (or Not Specified) as it was never explicitly set. I’ve added cycle access back again too.
With regard to the ‘Allowed Access’ setting on the editor I mentioned, for ‘All’ it was set to ‘yes’ but having now understood the bridleway issue I can see that is purely the legal position rather than the mechanism of access. In essence bridleways need to specifically allow foot access. If OSM updated at some point in the past to allow bridleways without having to specify foot access it may be there are many bridleways with unspecified foot access and a presumption of access would be far more likely than not I suspect.
How long might it take for my corrections to the OSM data to ripple through to your routeing engine?