Yes, it is definitely publishable. One prominent example from recent times is the resolution of the sensitivity conjecture in discrete mathematics/theoretical computer science.
It is sometimes the case that a conjecture is considered important not just because it has been open for a long time, but instead because experts believe that a resolution to the conjecture will use new methods or techniques that will prove fruitful in solving other related problems.
So, while it's great that you've managed to settle an open problem in your field, moreover one that has been publicized by an expert in the area, this alone may not be enough to have it published at a top journal in your field when your solution is elementary.
However, that does not mean it is not publishable anywhere. It would be best if you can contact a senior researcher in this field, say in your department, who can evaluate your work and suggest which journals have a high probability of accepting your submission.