To answer that question, we need to make an assumption that brackets will continue to be 10-19, 20-29, etc. It's likely that they will continue this way, but no guarantees. It's also likely that with the continuation of segregation of XP-on and XP-off queues, F2Ps and expansion-capped 50s will be the only active twinks to speak of.
With all of that said, if 19s ever plan to return in SL and beyond, I found three items that could potentially become grandfathered for 19s:
The
Blade of Eternal Darkness makes for arguably one of the best twink caster weapons in the game. Dropping out of Maraudon, the instance is currently available to characters in their 30s and above, and will very likely remain out of reach of SL 19s once the SL prepatch lands.
From a quest item that drops in Ferales, the
Chain Link Towel remains a BiS overstatted caster reward for 39s even if the quest doesn't proc rare or epic. It's very possible that when the SL prepatch lands, the downscaled version will still outdo any possible dungeon drops at 19, and the quest will stay out of reach of SL 19s.
Gwenyth's Wristguards see the same situation (the quest offers leather and mail variations for both factions). They're so good even without a rare or epic proc, they took BiS for 39s, 49s, and 59s in BfA. A product of a long questline in the Hinterlands, only a socketed wrist could compete...except none of the new dungeons coming for 19s offer a socketed wrist option for leather or mail wearers.
There may be more; these three items stood out as well-known pieces from the 39 bracket. If you want to dig for more possibly grandfathered treasure, check out the old
39s BfA BiS Handbook and see what you can find. Note that BoE items with procs often saw the procs not actually work at lower levels.