Side note: never seen a door like that in my life. Is it a regional thing or an old fashioned thing?
If you mean the mail slot, it's perhaps old-fashioned, since it's a security weakness.
I remember seeing an old movie where a family kept the house key on a string just inside, so when the kid got home from school they could reach in, get the key, and unlock the door (parents were at work).
And a couple where someone put fire or explosives through the slot.
As the other person said, having a house or flat (apartment) door with a slot in it is probably the most common mail receiving method in Britain and maybe other European countries too. Ireland almost certainly, and common or not, I've definitely seen them in pictures of the Netherlands.
Some buildings might have a dedicated separate slot, but due to convention it's often very near the door, and any external mail boxes aren't far away either. They're often on the wall next to the door.
I'm in California and the house I grew up in in the suburbs had a slot like that. I was kind of jealous of friends who had a "real" mailbox and going out to check the mail was a thing they got to do every day. I live out in the country now and the mailbox is 150 meters from the house so I only check it once per week. If there's a package that's too big to go in the box the letter carrier has to drive all the way up to the house and gets pretty pissy about it, they definitely would not come shove letters through the door every day.
I call my cat "muffin sweetie" when I think nobody is listening. Sometimes people are listening, though.
People are embarrassed by this?
If there's any consolation... This stranger on the internet loves that you sing to your cat and how much you love them. Absolutely no shame in that.
I sing to my dogs all the time, they each have their own songs that get screamed at them and they love it.
It's mostly in Finnish, but there are some translated ones I guess
