Planning niches for cabinets, wardrobes, etc.

Well, it usually revolves around the 60cm. You can, to stick with IKEA, look at PLATSA. There, the deeper carcass is 55cm deep and with door accordingly 57-60cm. Metod, actually IKEA's kitchen cabinet system, is also used by many here as a wardrobe/coat cabinet, has exactly 60cm depth, with front accordingly 2-3cm more.

Exactly, it revolves around 60cm or less than 60cm depth for such a niche is nonsense. But should we now have the niches made with 60cm, 63cm, 65cm, 66cm or what?

I find that difficult.

That might also depend on the niche?
For our Pax in the bedroom, we have already planned for the 70cm depth, but not in a niche either - you also need to be able to stand in front of it to take something out.
In the utility room, however, we have only planned 40cm depth up to the door on one wall because we know that we want to place kitchen furniture with 40cm depth there for storage.
Aside from wardrobes / PAX wardrobes, many pieces of furniture are only 40cm deep, but it also depends on the furniture you want to put in there. Therefore, there is nothing standardized.
We have made sure to have a bit of buffer with the depth in some places. After all, you eventually want to buy other furniture as well.

Well, do you already know if you absolutely need a sliding door? People either do it because you can't accommodate the projection of a hinged door in that spot or because they like the look. Otherwise, if you have the space, take 65 cm. You’re also supposed to leave a little space behind tall cabinets, sort of like a ventilation gap...

Is there a (non-)standard size for such a niche?

Yes and no. Yes - but useless to you if someone advises you to go for 63 cm, five heads (62.5 cm). And no, because plaster and tolerances do not simply add up linearly, and tolerances from verticality are often "forgotten" to be included. A few centimeters "too much" can be overlooked, I would probably take 70 cm as the rough construction target dimension.

A few centimeters too "much" look away, I would probably take 70 cm raw construction target dimension.

Yeah, a little more space is probably better than too little. But in some places, it might not look so great.

Do you mean 70cm raw construction without plaster? For us, it's about drywall. There won't be a thick layer added...

Do you mean 70cm shell construction without plaster? For us, it's about drywall. There won't be a thick layer on top...

Yes, that's what I mean. Is the niche surrounded "all around" by drywall, i.e. no masonry wall "involved"? - then the tolerances are less critical. In that case, I would take a whole board width plus x. Since narrow strips of drywall panels are a nasty hassle, the result would effectively also be 70 cm for a panel width of 625.