networking.wg-quick.interfaces.<name>.peers.*.persistentKeepalive

This is optional and is by default off, because most users will not need it. It represents, in seconds, between 1 and 65535 inclusive, how often to send an authenticated empty packet to the peer, for the purpose of keeping a stateful firewall or NAT mapping valid persistently. For example, if the interface very rarely sends traffic, but it might at anytime receive traffic from a peer, and it is behind NAT, the interface might benefit from having a persistent keepalive interval of 25 seconds; however, most users will not need this.