danielbachhuber
commented
6 years ago @mrwweb Do you want to submit a pull request against https://github.com/danielbachhuber/gutenberg-migration-guide/blob/master/tinymce-filters.md with suggested changes?
The TinyMCE styleselect is a way to define wrapping markup or classes to all or specific elements in the editor. For reference here is the TinyMCE and WordPress documentation for using the feature.
It appears that this function will continue working in the Classic Editor block, but I think documenting the overlap of it and Gutenberg may be useful and appropriate for this guide. The ability to set wrapping markup is a defining feature of blocks and the ability to add classes is now often covered by the new style variant feature in Gutenberg which allows defining and applying CSS classes to blocks.