I'm not so sure that signposting an unrideable stretch of bridleway for cyclists provides value for anyone, other than sign manufacturers.
The possibilities for sign manufacturers is endless, and the local authority would quote ludicrous amounts to temporarily close the path, make a risk assessment, give written notice to all concerned, set up a diversion, dig hole, plant a sign, and then reopen it.
Just look at the nonsense created by Covid cycle lanes and Councils closing roads to stop “cars” spreading the virus. Can a car catch Covid?
These people are brain dead from the start. No common sense.
And what would the warning signs say?
Hole in the road?
Beware dogs?
Red light district?
Beware of cars?
Beware of drug addicts?
Badly council maintained road?
.......the list is endless...