This module enables per subsite menu sets using Heyday's Menu Manager module.
Install via composer:
composer require guttmann/silverstripe-menumanager-subsites dev-master
No other configuration is required. Once installed any menu sets created will be linked to the currently selected Subsite at the time of creation.
Menu sets can still be accessed from templates via
$MenuSet("Your Menu Set").MenuItems but if you want
to ensure each Subsite only accesses its own MenuSets you can use this in your templates instead:
<% loop $SubsiteMenuSet("Your Menu Set").MenuItems %> <a href="$Link" class="$LinkingMode">$MenuTitle</a> <% end_loop %>