Task Constraint Notifications

There aren't native notifications for tasks only with a specific constraint, but here's a hacky idea you could do instead for this user. Create a task report with filter for 'assignment users' of that user's name (not 'assigned to' as that will only pull primary assignee), task constraint = must finish on, and any other filters needed. You can then create a recurring report send to the user at a cadence they want, i.e. twice a week. This won't send only based on their task's due dates, but if they want extra reminding about these must finish on tasks, this specific task report and recurring send could be helpful.