Hi Jonathan,
Welcome to Q&A.
We mainly deal with general issues of Excel desktop application. You issue in more related to Excel for web which is not supported in Q&A now.
Currently, Microsoft Q&A supports the products listed over here: supported topics (more to be added later on).
To help you better, I would like to suggest you post new questions into the forums below. Thanks for your understanding.
Besides, I personally do a test on Excel for web, indeed, there is no Notes feature, I just find Comment feature which allows you to have discussions with other people about the data. If needed, you may try to create comments. More information about the difference between comments and notes, please refer to "The difference between threaded comments and notes"