IFTTT support said a couple of months ago that their original integration was Groovy-based, and they were going to have to rewrite it for the new architecture. 
Based on what @Paul_Oliver has written below, it sounds like the new integration is working, but you are going to have to link to the new integration from the Ifttt side and you may have to re-create your applets in order to get them to run. There are some other differences as well.
OK. Thanks for the guidance.
The IFTTT integration has been updated. I switched mine over last week.
Not sure you read the whole thing. From the IFTTT side, it will not allow the re-link. It knows that my Smartthings hub location name is “AZ Home”, but when I authorize it to have access to the location, it returns the error “Could not connect to service”. Since I can’t get reconnected to ST (linked), I can’t access any of the SmartThings devices to recreate the applets.
After you got the error message did you go back and check Linked services in the ST. App?
For 1 of my 2 locations I got an error message when I tried to link, but when I went back into Linked Service it was in fact linked and my devices were available in IFTTT.
My IFTTT account fails to link as well. I contacted IFTTT support. Here is their reply
January 8, 2023
Hi there,
Thanks for reaching out.
We’re currently investigating an issue that is causing a small number of users to be unable to connect to the SmartThings service.
We’ll update you on this thread as soon as we have some additional information. Thanks for your patience while we investigate this issue!
TTThanks,
Daniel from IFTTT
Thanks for that update. I haven’t had any luck getting a hold of IFTTT.
I did go back and check, multiple times, and no linked service for IFTTT shows up in my list of services.
This is what I got from IFTTT support
There is currently a bug that is affecting some users preventing them from connecting to V2 if they previously connected to the existing Smart Things service so unfortunately it looks like that is causing the issue here.
TTThanks,
Dan from IFTTT
I would just start by creating an IFTTT Applet and chose SmartThings as the trigger and it should take you to linking your account.
@Paul_Oliver That’s what I did. they don’t have the service. It’s gone 
Doest it come under a different name than “smartthings” ?
I just tried it and had the same problem. I would bet they temporarily shut it down to work on the problem some people are having.
FYI I can confirm IFTTT Smartthings (Legacy) service still working OK.
Just tried it again. The SmartThings service doesn’t even show up anymore.
WARNING - DO NOT REMOVE YOUR EXISTING APPLETS OR UNLINK THE IFTTT SERVICE FROM SMARTTHINGS WITHOUT READING THE ENTIRE THREAD!
If it makes you feel better, it seems they removed the ability to use Smarthings in new applets even for folks that had successfully linked to the IFTTT API Access…
Yeah. I was encouraged when joe.asmar said it was working for him. I have, thru other channels, been able to get the service to appear to link back up to my SmartThings account. But in the IFTTT applets, none of my devices show up for actions.
Seems like smartthings was removed while they are working on it. https://ifttt.com/smartthings_v2 returns a 404.
Meanwhile
We don’t have an ETA for a fix just yet, but our engineering team are investigating as we speak, and you’ll be the first to know when this has been resolved.TTThanks,
Daniel from IFTTT
Still not on the app. Really unfortunate…
Yes I can confirm legacy still works, however, disconnected an applet by mistake and now won’t reconnect, I assume its because of the the issue above.
And as stated before, linked services does nothing appear in my account for ST nor does a search for it bring it up in the search results
