TWRP 3.3.x can not decrypt anything on the Fairphone 2 as of now …
However, as only the data partition gets encrypted, you can happily update your OS nonetheless, and backup and restore your OS, even if you encrypt “the phone” (= the data partition).
TWRP doesn’t make a complete backup of the data partition anyway, the Internal Storage gets left out on purpose …
If you want to try to decrypt, you can try your luck with a custom TWRP 3.2.3 here …