Thank you for watching, voting, and commenting on this issue.
Running multiple sprints in parallel from a single backlog is something we will bring to next-gen projects soon; although we have some other priorities ahead of it - including workflows, improved filtering, and time estimation. It's not possible to give a date but I expect this to be in the 6+ timeframe bucket.
In the short term we recommend teams use classic projects when you need to run parallel sprints. We apologise for the inconvenience this may cause you.
Please continue to vote and in particular please continue to add comments on how your teams find value when running multiple sprints in parallel from a single backlog. This contextual information of what you are doing and why you are doing is invaluable for us in providing you with a product that fits your needs.