[parasense]

> If someone sold me support for a piece of open source software (like ISC and BIND) then I definitely would expect some level of ...

The word you were looking for is "support".

If there is something wrong with the "supported" open-source software, then you may expect a certain level of "support". Full-stop.

That generally entails an SLA that says your issue will be reacted to within N-time of opening the issue, which might be nuanced by the tier-level of support purchased. That you are provided access to documentation, or even the source code itself. You might be provided with best-effort support by an agent, which is limited to resolving documented defects, or configuration, or acknowledging standing-bugs which cannot be resolved.

What you cannot expect is the software is updated in accordance with the support incident. For that, send patches, or pay somebody to send patches.