@Mantis - welcome to the forum! Advanced search is available on paid plans; did you recently switch from a paid to a free plan? If not, could you post a screenshot of what you see in the search bar?
I do agree that it would be helpful if a collaborator view was in a more prominent place (like My Tasks). Consider voting for this thread:
@Mantis - In addition to the helpful guidance from @Stephen_Li , it also might just be that your account was recently moved to the new Advanced Search button.
It used to be an option at the bottom of the search dropdown.
But, now it’s a toggle at the right side of the search bar when you click the search bar