Recent publication and special issues
To the best of my knowledge there is no way to get MDPI to notify you of upcoming special issues without knowing about everything else they publish.

If you're looking to submit to those special issues, I recommend looking for subject-specific communities (e.g. mailing lists or similar) that people might advertise calls for papers to.

If you're simply looking to read them, I suggest setting up some good Google Scholar alerts around the subjects; if you get these right then as well as giving you interesting articles from all over the place, they will tend to show up multiple articles from the same special issues.