Can't set route with VPN post script since upgrade to Big Sur
Same here ... it was working for a while though.
What seems to work is:
networksetup -setadditionalroutes &lt;networkservice&gt; [ &lt;dest&gt; &lt;mask&gt; &lt;gateway&gt; ]*

To find out the adapter names use:
networksetup -listallnetworkservices

Example:
networksetup -setadditionalroutes &quot;name of your VPN&quot; 192.168.1.0 255.255.255.0 192.168.10.254 192.168.2.0 255.255.255.0 192.168.10.254