As far as I understand, the Chultan Peninsula became an island for a time during 4e, and after the Second Sundering the regions around the Mhair Jungle were restored. Tashalar, an established and prosperous nation on the peninsula, was described in earlier editions (2e and 3e if I'm not mistaken), then presumably disappeared in 4th edition.
In 5th edition, several neighboring nations on the Peninsula are mentioned in passing in the Sword Coast Adventurer's Guide (p. 10), under "Lands to the South":
Eastward along the Chultan peninsula lie the remains of Thindol and
Samarach. Despite the apparent fall of both civilizations, Thindol
remains infested with yuan-ti, while the illusions cloaking Samarach's
mountain passes conceal the activities in that nation.
I've yet to find any reference to the current state of Tashalar, aside from a mention in the Forgotten Realms Style Guide under the heading "Don't Whitewash the Realms":
The Forgotten Realms is a huge world with many earth analogs. Turmish,
Tashluta, and Chult provide sources of humans with black skin and
features.
(Curiously, as far as I can see in previous editions, "Tashluta" was a city in Tashalar, and not a nationality or a region.)
Is there any canonical reference to the current state of Tashalar in 5e?
I'm exploring basing an adventure to be published on DMs Guild in the region, so I'd like to avoid contradicting anything published. I've seen the Forgotten Realms Wiki entry and the Realmshelp listing, but as far as I can tell, both refer to earlier editions of the game.
Best Answer
As of writing, not that I can see
From page 120 of The Grand History of the Realms, the 3.5 edition sourcebook, we know:
This, presumably is why we don't hear much about Tashalar in 4th edition.
I went through the following sourcebooks to see if I could find a reference to Tashalar by name or implied reference. Here is what I found:
Acquisitions Incorporated contains an adventure set in the Dessarin River Valley. It contains a map of the Savage Frontier that extends only as far south as Waterdeep.
Baldur's Gate: Descent into Avernus deals predominantly with hell. Waterdeep, might theoretically have some sort of trade with Tashalar but this is not explicitly stated.
Curse of Strahd takes place in Barovia, which is wholly separated from the Forgotten Realms. It is a demiplane located in the Shadowfell.
Dragon of Icespire Peak is set around Icespire Peak, which is in the Sword Mountains just north of Waterdeep. (The supplementary adventures Storm Lord's Wrath, Sleeping Dragon's Wake, and Divine Contention are also of limited scope, focusing on the area between and around Neverwinter/Waterdeep.)
Eberron: Rising from the Last War focuses on Eberron, a separate planet/realm from the Forgotten Realms.
Explorer's Guide to Wildemount focuses on the world of Exandria, an entirely different setting from the Forgotten Realms.
Ghosts of Saltmarsh is set in the Greyhawk setting by default, and therefore does not reference the Forgotten Realms (outside brief sidebars on setting the adventures in other realms, but Tashalar is not mentioned there).
Guildmaster's Guide to Ravnica deals exclusively with Ravnica, a separate plane from Toril.
The Tyranny of Dragons adventures (Hoard of the Dragon Queen/Rise of Tiamat) deal with the area just north of Amn up through the Savage Frontier.
Locathah Rising mentions the Sword Coast, Turmish, and Thay (and the Elemental Plane of Water).
The Monster Manual does not mention the region. I specifically checked through the section about yuan-ti in particular because their lore seems to intertwine with Tashalar's quite a bit and they are said to inhabit the area according to the SCAG passage you quote.
Mythic Odyssey of Theros is also a standalone campaign setting that does not intersect with the Forgotten Realms.
Out of the Abyss takes place in the Underdark. It touches on the surface world a little, but this is limited to the Sword Coast and Silver Marches.
Princes of the Apocalypse is centered around the Dessarin Valley, with references to other nearby areas of the Savage Frontier.
Rrakkma deals with Pandemonium, the plane of madness.
Storm King's Thunder does not reference anything South of the Savage Frontier.
The Sword Coast Adventurer's Guide*: In addition to the excerpts you mention, on page 115 it reads:
The "coast of the Shining Sea" would include the region of Tashalua, and so we can assume that there are a number of Rock Gnomes in the area. Not super illuminating.
On page 44, there is also a reference to a family whose ancestors hail from Tashalar:
Tales of the Yawning Portal is a collection of adventures from D&D's history. Several of the adventures are set in the world of Greyhawk, and almost all of them are designed to be location-ambivalent so that they may be dropped into other campaigns.
Tomb of Annihilation does not mention either Tashalar or the city of Tashluta. The map on page 39 does not extend past Samarach and the Sky Lizard Mountains.
Volo's Guide to Monsters does not mention the region. I again checked the section on yuan-ti in light of their history with Tashalar and SCAG's mentioning their presence in the general area, but I could find no specific reference.
Waterdeep: Dragon Heist: Waterdeep might theoretically have some sort of trade with Tashalar. There is nothing to confirm this but there is a family of people from Tashalar living in the area per page 13 and it could be that they still communicate with their homeland as part of acquiring their monsters:
Xanathar's Guide to Everything is a sourcebook that does not describe any specific setting.
In short, it seems that there are pockets of Tashalar immigrants living in the Savage Frontier but the region itself is a bit of a blank canvas when it comes to 5e.