My question is it's become a thing to name companies with real words like Slack, Lime, Bird... What are the copyright ramifications? When we started with Cake there was the band in Sacramento, Cake php, but nothing that seemed major in our space at the time. While we were in development some other firms adopted the name.
Do you advise companies to pick something more like Airbnb?
Your question is more about trademark law than copyright law. Those are all pretty decent names because they're arbitrary. The USPTO looks at trademarks on a scale including fanciful (XEROX), arbitrary (APPLE for sale of computers), suggestive (COPPERTONE for sunscreen), and descriptive marks. Fanciful marks are the strongest and are made up terms and arbitrary marks are the next strongest where the mark is unrelated to the goods and services offered.
The marks you picked are all pretty good because they're all arbitrary. I always advise people to lock down their trademark rights early. There's nothing more heartbreaking that building a strong brand only to discover someone else filed a trademark application and now there's an urgent need to rebrand.