Just wandering, what http3 solutions are on Linux side. Can IPVS be http3 compatible at all?
Why not? As long as you can send and receive UDP traffic, there's no reason why you can't do http/3.
Caddy and HAProxy both support the protocol. nginx doesn't have it in the public stable version yet, though there are preview packages available for modern server platforms.
HTTP/3/QUIC supports migrating connections between two networks, such as if a user switches from WIFI to LTE. IPVS or any UDP load balancer won't handle this scenario properly since it doesn't introspect the QUIC header and load balance based on the QUIC connection ID. This QUIC connection ID allows for a stable connection when the device needs to switch networks. If operators have any sort load balancer (like IPVS) between the client and the point the HTTP/3 connection is terminated, they will need to ensure that it has proper support for QUIC. One example is Katran[1] which has support for this method of load balancing.