I think Ordnance Survey have given up on updating their maps with new footpaths. Possibly because there are some legal/official restrictions on what can be called/designated a ‘footpath’, possibly because they’ve made a decision not to clutter their maps with new stuff.
OS will add a new unmade path (a black dashed path on OS 25k mapping) in undeveloped land if a robust case is made to them that a) it is in the public interest to add the path and b) there is good evidence that the path exists on the ground.
The PI test has the best chance of being accepted if the path joins parts of the PROW network and goes somewhere specific e.g. a trig point, a historical feature etc. i.e. the reason the path has developed. Evidence of there being a visible path on the ground can be as simple as a screenshot of satellite imaging annotated to clearly show the line of the path along with photos taken along the route (A GPX track is also helpful.)
Here are a few examples:
A few years ago, I had a dialog with OS about the waymarked and signposted YDNP/NT permissive path that traverses Addlebrough in the Yorkshire Dales, which was unmapped. Given the rationale and underpinning evidence I supplied, they added it. Not quickly, but they did add it.
Another example is in Wharfedale and is the black dashed path leading from Mossdale SE 0209 7011 up to Sandy Gate SE 0297 7120, which was also added recently following my request. The line of the bridleway shown may be the legal right of way, but there is no path on the ground and the dashed path shows the route as waymarked by YDNP and is clearly visible.
OS also added a missing track and footbridge, and added a new wood including its fire break and added its name (Millennium Woods) near Lauder in the Scottish Borders, again after making the case that these features should be mapped.
I hope I have made the case with the above examples (there are more) that OS can and do update their 1-25 and 1-50k maps with unmade paths in undeveloped land - it is up to us to make the case for adding them and provide them with the evidence.