don't know if that is possible in Windows server
It is. Option 249 is Microsoft's proprietary alternative to Option 121. The difference between the two is that Option 249 is used
in addition to Option 3 (list of default gateways), whereas Option 121
replaces Option 3 (i.e. it contains the complete route list, including the default one).
i don't understand idea of route with br-loopback as the gateway.
First, you only need this if the Option 249 approach fails.
Second, an interface name can be set as a
gateway parameter of a route. If it is a point-to-point interface, it always works; if it is a point-to-multipoint interface, it only works as such if the underlying network stack accepts ICMP router advertisements, so it learns a list of routers accessible through that interface. This doesn't seem to be the case of RouterOS for IPv4 (or at least it wasn't when I tried that last time years ago, and the manual explicitly states that it doesn't work). However, the fact that it doesn't work does not interfere with the possibility to specify a P-MP interface as a
gateway of a route.
If it was a real Ethernet interface with a router connected to it, the packets would be dropped, but in your case, the IPsec traffic selector will match these packets before they actually reach the interface, and will divert them to the IPsec SA. So you can reuse br-loopback as a gateway of a route shadowing the default one for the destinations reachable via the SA. The default route points to a router in the same subnet from which the PCs send their traffic, and would thus cause the ICMP redirect messages to be sent to the PCs.
Another application of the same approach is a "killswitch" for IPsec VPNs - if you want to prevent traffic that should only ever be sent via a VPN from leaking through the default gateway when the VPN is down, setting a route for this traffic via a bridge with no physical interfaces does the trick. A
type=blackhole route doesn't because blackholed packets are dropped before they reach the traffic selector matching phase.