Jeroen 🏳️🌈
[Mister Know All-mode on]
I think spelling rules to always start a name with a capital, apply to all possible names. This has a reason: It increases readability as it helps the reader to define an unknown word as a name.
All caps is used in abbreviations for names that consists of multiple words. Like ANWB and FBI. But note that you pronounce all letters here. Ikea therefor should be written Ikea, as it is a name and pronounced as one word (it also is an abbreviation (founded by Ingvar Kamprad from the Elmtaryd farm in Agunnaryd village), so you could write IKEA, but this means you want all letters to be pronounced [ai] [kei] [i:] [ei]).
Everybody and every company is free to choose their own name, of course. I know a lady from Belgium who signs her letters with Cat'rina. Maybe she finds Catharina old fashioned, or just wanted to do something cool. But I doubt this is in her passport and she can't force official institutions to address to her in such way. This goes also for companies, of course. They can write lowercase and even CaMeLcAsE if they like, but this is nice for their brochures and website. It doesn't mean all people in the world should start breaking spelling rules as soon as they want to write about or to this company.
There are exceptions: in some cases many editors and writers follow a company's own way of writing, if they want to increase readability (like with ArcelorMittal instead of Arcelormittal (lol, even my spellchecker suggests a capital M) or with iPhone instead of Iphone). But writing bunq decreases readability, so that's not really a valid argument here.
[Mister Know All-mode off]
On this forum, everybody is free (and welcome) to write how he feels, of course. It was just to explain that Bunq can't rewrite spelling rules and Sander can't tell me to break them :-)