How safe is to use the 120-126 extension ID range?

Based on version 5 of the IETF draft, when choosing extension ID, my take is that an application using Opus should not choose 0–2 or 127. It should also not use 3–119, unless it's intended for broader interoperability or standardization, in which case it requires to publish an IANA registration request.

Then 120–126 is the only defined range in the draft where we can choose IDs for private or prototype usage.

However, these are officially “experimental,” which could mean that:

1. They should not be relied on for long-term interoperability.
2. Two independent implementations using experimental IDs might collide unless coordinated.
3. A future RFC may redefine usage and require code changes.

If today an application uses, let's say ID=120 for a custom extension. I understand that other applications can be using the same ID for a different purpose. And that's fine, because they wouldn't collide with my application's scope.

Within the scope of my application and its user base, how safe is to stick to an ID from the experimental range without taking the risk that future Opus lib versions could repurpose it? Assuming a case in which we are not planning to publicly standardize our usage.

Another way to frame this question: Instead of "experimental", can they be declared as "private-use", and guaranteed to remain that way in later iterations?

Read the linked version 5 IETF draft on the Opus extension ID range and compare its treatment of IDs 120–126 with the issue’s private-use question. Check whether the project documents any compatibility guarantees for these IDs. Done means an authoritative clarification of their long-term safety or private-use status.