Are any types formula fields able to be encrypted via platform encryption? I have been looking for options, and so far, I have been unsuccessful in locating any documentation. If not, what have you been using to encrypt said values?
Formula values are not stored in the database, and thus cannot be encrypted. Instead, formulas are generated the moment a query is performed (e.g. to view the record, in a report, etc). If you wanted an "encrypted formula", you'd have to use a custom text field, encrypt that field, then use a workflow rule/Process Builder/trigger/etc to specify the value to store. Note that cross-object formulas would behave oddly, since they could only be calculated the moment a record is created or updated.