What happened is that I fixed a bug in the s3 backend which wasn't reporting errors on bucket creation properly.
This then surfaced the fact that people like yourself had been depending on that bug!
The no_check_bucket = true
means that rclone won't attempt to see if the bucket really exists or try to create it.
That is to work around a limitation of the SDK, but it shouldn't make any difference.
If you don't want to use that flag, then rclone would like to be able to create buckets. If you don't want that then no_check_bucket = true
is the workaround.