For edge cases such as:

https://bugs.openjdk.java.net/browse/JDK-8218123

it does not make sense to complicate the logic of the sync bot (in this case it would need to understand that there are some 11 releases which used b31 values value 11.0.2.0.n).

The idea is that if the bot sees the label it does not try to check the backports.
There was an existing bug with a hgupdate-sync-ignore label but that does not appear to be checked as it was added to JDK-8218123 and the bot still updated it incorrectly.