The 1.15 release branch cut is planned for next Monday, 8/1.
After that point, only changes cherry-picked onto release-1.15 will be included in the release, and barring some last-minute exceptions (please reach out to #release-1_15 for these), no new features will be merged.
For more information check out https://github.com/istio/istio/wiki/Istio-Release-1.15.
Thanks and happy Istio-ing,
Sam, on behalf of the 1.15 Release Managers