Don't know where you getting your info from, but the FGT/WF-20C-ADSL / FWF60CX-ADSL-A is still a sold and supported item. In fact the v5.2.x has code out for both of them. I've also installed 2 in the last 2 years. They are quite expensive when compared to the native model but they do work great and the ability to have a 3g-dialbackup and/or wan backup is great.
It would be nice if a SMB model was available with a WAN expansion slot and a ADSL card interface. That would be extremely nice to retro fit a SOHO model but that might be pricy to support from a vendor side of things.
BTW: FTNT doesn't list these under the product page since they are low demand items, contact your fortinet dealer straight for pricing.