Section 31 (6) of the Highways Act 1980 enables a landowner to deposit with the highway authority a map and statement showing ways which they admits are highways, and if they then, within 20 years deposits with the highway authority a statutory declaration that no additional ways have been dedicated, this is sufficient in the absence of proof to the contrary to establish that no additional ways have been dedicated. They can then continue to deposit similar declarations every 20 years, with the same effect.
Essentially, it’s a means of stopping rights of way coming into existence by virtue of 20 years use.
The notice has no effect on rights of way which are already securely recorded on the definitive map. However, the effect of the deposit of a map and statement is to challenge the use of any routes not declared and so this is the time when an application should be made to get those routes shown on the definitive map. If you are relying on evidence of public use to prove the existence of any of the rights of way, then the requisite twenty years use would be counted back from the time of the deposit.