I can't speak for Dexron, although I think you're right about the older ones being deprecated by GM. SL is still a current specification under the API. GF-3, however, is not. So, if an oil shows SL and the API Donut, it's legitimate. If an oil shows SL and the Starburst, it's either baloney or very old stock. I'm sure I could find some old stock here if I looked hard enough.
You'll see SL legitimately used on European A3/B4 30 grades, such as GC 0w-30 and Castrol Edge 5w-30 A3/B4. The A3/B4 40 grades get to use SM or SN, as the case may be. They couldn't sneak in under the old CJ-4/SM or CJ-4/SN rules for phosphorus, since A3/B4 would have a difficult time qualifying for CJ-4 or CK-4 due to excessive SA. So, we see an SL for 30s and SN for 40s.
Of course, there are oils out there that do just claim SL or newer, and do so honestly, without an attempt to misinform. Euro oils have claimed to meet the wear protection requirements of SN (obviously they do and exceed that) but not the phosphorus levels. Some very niche HDEOs from Imperial Oil (i.e. Mobil) and Petro-Canada claim suitability for certain API specifications, diesel and gas, without actually being formally certified, either due to it not being worth certifying or the certification being obsolete. Petro-Canada data sheets are very clear as what HDEO is actually certified for what specification and what it's actually just suitable for use.
That's most commonly seen with an oil where you see CF on it and can trust it to really be suitable (i.e. M1 0w-40, RP 10w-40). Those oils would be very legitimate (if costly) choices for a fair number of diesel engines still on the road, such as old Cummins, Powerstroke 7.3, GM 6.2 and 6.5 examples, and various older agricultural engines. In fact, they'd be much more suitable than various oils of the day that tacked on a CF. You'll also see reference to an obsolete certification when you're looking at the dedicated two stroke diesel engine oils.
As an aside, back when CF was current, my dad decidedly did not use a PCMO with CF tacked on in his diesel engines, even though that was technically legitimate. 10w-30 was a suitable winter weight for most of them, but he didn't grab QS 10w-30 that happened to have a CF on it. He bought Esso XD-3 in the appropriate variety and ran it. This was a weakness of the API specification at the time, and those who didn't see it could face issues. Doug Hillary has also told us some very interesting historical details of how that transpired.