A friend of mine just recently completed his driving test. Luckily for him, he passed, but just about. One of the bad marks he received that had him close to failing was for “misleading”. He received the bad mark for thanking someone for letting him out of a junction. It was misleading of him to do this according to the driving test.
I was aware of not being allowed to do this when I did the test myself, but just hearing that he was close to failing the test for this, really got me thinking about it.
The test for me has always been about making us drive like robots. In reality, the only time people drive like this is in the driving test. If everyone went around not thanking people with gestures for common road courtesy, what would road rage be like then?

I failled my test about 10 years ago with a mark against “Do not beckon other road users onto the road.”
I guess they generalized that as it seemed very specific to me.
When I went for lessons the next time the instructor told me that waving people on, saluting or other non standard hand signals can be confusing and have caused accidents in the past.
I too have seen it causing accidents, but it’s generally the other person being really silly.
For example, you let someone pull out in front of you and they just drive without looking to see if anything is coming from the other direction - that’s their issue and not yours.
I just think it’s ridiculous that they expect you not to say thank you or something or the sort as for me that just makes my blood boil!