how to completely remove horse's records ???

"You can also choose to send a horse for an early retirement by donating it to a riding school. This will permanently remove the horse from the game. Its records will be kept, unless you decide to manually remove them as well".

I thought that manually removing records meant that a horse becomes invisible in the pedigree. I paid 300evd to retire it, then went to "former horses" and clicked "remove records" and yet, this horse is showing in my mare's records. Basically it's the same  as if I'd dumped it in RC (for 200evd) and nobody had collected it from there. I thought the the idea behind paying 300evd was that the horse gets erased completely ?!
What am I missing here ?

You used to be able to do that but it has been changed. It was patched this way to prevent broken pedigrees in bred horses I believe, and also prevents players circumventing the inbreeding block. It sounds be working as intended. 

Very disappointing ! Then I believe the description should be changed because it's causing misinformation.

What would you suggest it was changed to? It doesn't say that the records will be wiped from the game, just that it's records will no longer be tied to your name/the account it was retired on. 

It does do what it says, however a horse you send to an early retirement will still show in your former horses. You can remove them from your personal records(former Horses) by hitting the remove records button at the top of the page. However the horse's information will remain intact to prevent broken pedigrees if it has any foals.

What you are thinking of is the old delete horse function which did used to completely remove horse records- that was not carried over to this version. 

The retire horse function was introduced in 2019 to replace that function, but has always been coded to save the records. You can view the newspost here: Feature Updates. 

Ok, so my memory must me misleading me, because I was convinced that this new retire function had the same consequences as in pre-recode times. Glad I had it explained !
Still, I'd argue that a sentence: "Its records will be kept, unless you decide to manually remove them as well", strongly suggests that it is possible to manually remove all horse's records. Even though in reality manual removal merely means that nothing but horse owner's name is removed from the records. So this is the sentence which I find to be misleading and which I would like to see adjusted.