If you have DKS on Steam, you should now have the Developer's Cut version in your game library. Beyond Divinity is also listed on Steam, in addition to the Divinity Anthology. Was BD itself listed as "special DLC" or was that something else? If the former, then before the official Steam release today BD was probably only available to those who had a code for it (included with the boxed anthology).
Earlier games have not been included as DLC with previous Divinity games, though some retail releases of Beyond Divinity included a copy of Divine Divinity. BD is available from GOG, Steam or the Larian Vault, though.
Speaking of DLC, the codes for D:DC and D:OS items are only included in the retail anthology. However, there was a Chinese New Year
promo offer for Dragon Commander DLC early this year. The
webpage is still up, so you can try submitting your email address to get in on the deal (as long as you don't mind silver instead of gold). I have not gotten any announcements from signing up (despite leaving the option checked to get updates), so you don't have to worry about getting on any mailing lists, etc.