I don't think the thank you notes are a sign of the market shift. I started seeing them from people coming fresh out of code camps, and assumed it was just some jobs-hunting coaching recommendation.
Since then I've seen a few more, but not many. I don't think I've ever waited long enough after an interview to meet with others on making a pass / offer decision for a thank you note to make a difference. Even when we had a group for a single position and there was time to pick between candidates, we at least had already passed on our feedback by the time one could be sent. At that point, I don't know that a thank you note would make a big enough difference for me to reach out and change my feedback that would put them above someone else in consideration.
Back when I was working for a company that did take home challenges, those who passed had their challenge as the topic of conversation for the final interview. For those who didn't, I gave written feedback, at least a page if not two.
My thinking was if someone took time to give me something, they deserved something in return, even if it wasn't a job.
If you have not been through the process you may not truly understand how nice (and rare) your thought process is.
Sadly I have chased up situations like that only to get told "you provided one of the strongest pieces of work, however, we decided not to proceed to interview" - or responding at all apparently.
This must be highly cultural. After you done the interview, shook hands and thanked the interviewer there in person there is little point of follow up other than an opportunity of reminding of yourself in the consideration process.
Every interviewer seems to have some set of these weird little expectations about behavior, that are basically arbitrary. Worse, some are mutually exclusive (maybe not so much in this case—though, see elsewhere in the thread where someone mentions this seems like a fresh-out-of-code-camp thing, so it might send a negative signal!) so lots of rejections end up having a reason behind them, but a very stupid reason that you can’t really defend against, to be blunt.
Bullet dodged IMO. I'd rather work somewhere where talent, productivity and genuine charisma gets me ahead instead of ass kissing (yes, it's always ass kissing... These letters are never sent out of a genuine gratitude for being able to interview with you).
I have no problem with thank you notes; I think they’re courteous and easy to send. It never hurts. If you don’t get hired you lose nothing, and if you do, you’re starting on a pleasant note.
But I would definitely want to know more about the governance of an organization that can sustain such a policy as categorically rejecting a candidate (read: contribution) for not meeting an unexpressed expectation.
As a UK interviewer I'd find them both unusual and annoying.
We already discussed you immediately after the interview. It's not going to change anything, but it might hurt your chances if it comes across as cringy or brown-nosing - we're also looking for cultural fit, and most Brits just don't do that. If you introduced new information in the email, you're probably going to force us to discuss you again, which is at best annoying and certainly burns up staff time.
So in a UK context, you're best off just saying "Great to meet you all - hope I'll get to work with you" at the end of the interview, and leave it at that.
Since then I've seen a few more, but not many. I don't think I've ever waited long enough after an interview to meet with others on making a pass / offer decision for a thank you note to make a difference. Even when we had a group for a single position and there was time to pick between candidates, we at least had already passed on our feedback by the time one could be sent. At that point, I don't know that a thank you note would make a big enough difference for me to reach out and change my feedback that would put them above someone else in consideration.