You can control how visitors get in touch with walk leaders. This is set up in Admin → System Settings → Area & Group → Event Configuration.
There are three options for how the "Contact Walk Leader" link works:
When using the contact form, if a walk leader's email address isn't available, the message is sent to a fallback committee role (e.g. Walks Co-ordinator) that you can configure.
When walks are synced from the Ramblers Walks Manager, the leader may only have a Ramblers website URL rather than an email address. The Rematch button matches synced walk leaders to your site's member records and fills in their actual email address automatically.
The settings above control your own site. The listing on the Ramblers website is different: a leader's name, email address and phone number only appear there if the leader has given permission in their own Walks Manager contact preferences. The steps, and what Ramblers then show, are in Your Contact Details on Ramblers Walks.