FontForge's sfd (spline font database) format changes over time as fontforge supports more things. I have only recently started keeping track of these changes, so older changes are not listed here.
This also involved removing the old vertical origin keyword which is (I hope) a subset of the BASEline information.
All fonts may now have multiple layers (before only Type3 fonts could). The Type3 format has not changed so the discussion that follows concentrates on normal fonts and should be understood not to refer to type3 fonts.
All glyphs in a given (non-type3) font have the same number of layers so there is a LayerCount keyword in the font which provides that (there is also a LayerCount keyword now in each glyph. This provides redundant information, it should be the same as the font's layer count (for non type3 fonts).
Before this the foreground and background and Guide layers all had the same spline type (quadratic or cubic), now that can vary from layer to layer. There is a font level keyword "Layer" which provides the spline order and name of each layer. The old "Order2" keyword has been removed.
Previously only the foreground layer of a glyph could contain references, now any layer can. Before the "Refer" keyword could appear with no layer name preceding it (as this unambiguously meant that the references would go into the foreground layer), now there must be a layer name preceding a "Refer" keyword.
Each layer output is preceded by one of "Fore", "Back" or "Layer: d" (where "d" is the layer number).
Before the SplineSet keyword was not needed (and was usually omitted) after a "Fore" or "Back" layer name. It is now required, and this seems to render new sfd files incompatible with old.
(If there is no layer tag before a Refer keyword then the parser will apply it to the foreground layer)
Previously the validation status applied only to the foreground layer. Now every layer can have its own status, so again the "Validated" keyword must now be tagged with a layer.
There is a font level keyword "DisplayFont" which indicates what layer should be displayed in the font view by default.
There is a font level keyword "GridOrder2" which may appear before the "Grid" layer name in the font to set the spline order of the guideline semi-layer.