Bridleway cutoff date 2026

This isn't just bridelways. Includes the other types of right of way too (footpath, restricted byway and byway open to all traffic).

My suggestions: Encourage them to help map them in OSM. Meanwhile, I have sent this to the equivalent person at the Ramblers as they are looking at a crowd source project to pick up the footpaths before the 2026 deadline.

I agree we could and should encourage Ramblers and BHS to map what they have and what they think they should have.
I would be happy to help talk to BHS and Ramblers in my area. To show what tools OSM has and how they and I could record information. I would also help them enter the data. I suspect new phone Apps and renderings would be required to highlight the ROW's in the field. MapThePaths is an excellent starter from which to gather the data and help present it to the authorities and show to Ramblers and BHS - the overlays are dynamic and better presented than I've seen from OS.
If OSM could create a ROW group known to each other who could do this on a local or national basis to help coordinate activities it might be helpful to resolve common mapping issues and spread the word.

I'm not sure OSM itself is quite the right tool, but certainly the GIS/mapping experience of many of our contributors could be useful. My feeling is that what's really needed is some sort of national tool/platform that shows existing confirmed Rights of Way, and allows users to add potential unrecorded routes and add annotations with history and evidence that's been uncovered. Ramblers groups and others could then use this to help prioritise work on the most important missing links. (There could be some great publicity for OSM here if OSM-UK could be involved somehow.)

I don't know how useful this is as it stands, but as part of my work in OSM on mapping footpaths in Norfolk, I've been keeping note of any gaps in the network where paths don't meet up, or have dead ends: https://osm.mathmos.net/prow/progress/norfolk/network-issues . Some of these will be errors in the current recording, some will be legitimate as the result of a stopping up order, but I would expect quite a few will be because another route with historic rights hasn't been recorded. I had a go at trying to get some interest from the local Access Forum and ramblers some time ago, but without success.

I have some experience of this in my local area, where some land previously owned by the Council was sold off. The footpaths through the wood had been maintained by the previous tenant and the Council for a generation and the new landowner blocked them all off. The whole process was a nightmare and took months of constant badgering to the Council to get them added to the Definitive map. To help locals provide evidence I put maps and the evidence forms on my site although those maps were O.S. ones as at the time OSM didn't have the needed detail (I added it afterwards) and there was a time limit from the Council. If there was an easier way to submit evidence it would be great like @robertwhittaker suggests but I doubt the councils would accept it unless on their exact forms. A link to the start of my original page is here and afterwards.

If there was an easier way to submit evidence it would be great

Yes, this is exactly what The Ramblers are looking in to. Initial chats suggested that they hope to partner with organisations such as BHS. Last I heard the project was waiting internal sign off.