I know that @Legislation isn't supported by BibLaTeX. But it is mentioned in the documentation of BibLaTeX (as unsupported). So it is part of some special *tex-packages. This means it is needed by some users.
I am using 3.8.1. JabRef doesn't offer @Legislation. That is ok. I can modify the bib-file with a text-editor. But after reopening it the edit window (registar cards: general, abstract, ...) doesn't appear. This happens for any entry not only for the @Legislation one.
JabRef should follow the BibLaTeX docu and tread all unknown/unsupported @-entries as @misc. Sounds easy for me.
Maybe I missunderstand someting here?
If not I would open a Issue for that.