We as part of enabling OpenShift Serverless for s390x and
ppc64le architectures are actively working on knative upstream release
to keep them updated. The members here working actively are @dilipgb (for s390x) and @valen-mascarenhas14 (for pp64le).
With respect to istio envoy images - we leverage maistra/envoy packages (midstream of istio envoy packages) for testing knative functionalities. There is active work happening on maintaining maistra/envoy for s390x and ppc64le architectures.