Using the Image Provided, You must be signed up for the Microsoft 365 Insider. Next, you must select the "Beta Channel" instead of "Current Channel (Preview)". (It should be noted that you are able to go back and forth between these 2 channels.) However, If you use the "Beta Channel" and create the checkboxes, then go back to the "Current Channel" your checkboxes will no longer be there and will be replaced with a TRUE or FALSE value. Also, if you send the Spreadsheet to someone else that does not have the Beta channel enabled, the checkboxes will have the same result.
To my knowledge, the checkboxes will remain in the Beta Channel until it is determined they don't have any bugs or cause any issues.
ETA to Current Channel = Unknown
Hope this helps!