An interrogative whether clause in adjunct (modifier) function doesn't necessarily need "or not". Compare:
They will play tomorrow whether or not it rains.
They’ll complain whether we play tomorrow or at the week-end.
"Or not" only occurs in the first of those two examples; nevertheless both whether clauses are adjuncts (modifiers), more specifically ‘exhaustive conditional adjuncts’. The first means They will play tomorrow if it rains and they will play tomorrow if it doesn’t rain: They’ll play under either of those conditions. The second means They’ll complain if we play tomorrow and they’ll complain if we play at the week-end: They'll complain under either of those conditions. In both cases, the two conditions exhaust the options, hence the 'exhaustive' component of the term 'exhaustive conditional'.
Putting these videos on YouTube will be a good opportunity for
figuring out whether or not a wider audience will enjoy them.
This example is different from the other two, even though it still contains "or not", which is optional this time. It’s not an 'exhaustive conditional', and it does not function as an adjunct (adverbial) but as complement to the preposition "out". The whether clause is still an interrogative subordinate clause, though. The meaning can be glossed as “Putting these videos on YouTube will be a good opportunity for figuring out the answer to the question ‘Will a wider audience enjoy them, or not?”’