I wonder whether there exists a website that offers information about journals' upcoming special issues and 'call for papers' subject-wise. What are your suggestions?
I doubt that such a website exists. There are thousands of journals in the world, and it would be a very time-consuming to check all their websites about once per month. Usually, the number of journals in a specific field is limited, and you may regularly check their websites.
There is no such website exists to display the list of special issues in all journals. However, one can login into any journal management system and upon clicking the new submission, the author will be asked to choose the kind of submission. The pull down menu usually lists out the special issue name if exists. An example is attached.