I was browsing the directory for RTE add ons.
I tried TinyMCE and was pretty happy with it.
One of the other options I stumbled upon was the HugeRTE, whose landing page says
This add-on provides Java API for the HugeRTE rich text editor (an MIT licensed for of the now “GPLd” TinyMCE project).
So does that mean I need an API key if I want to continue using TinyMCE for commercial use?
In short: TinyMCE has changed its license to GPL starting with version 7. This means, for closed source projects, it’s not longer free to use. Idk atM if the Flow addon is alread on 7, but as soon as it is, the Tiny 7 license applies to it.
Tiny MCE 6 is not longer maintained, so for commercial projects also not an option, since these want most times only maintained libraries to be used. Therefore some people forked it a while ago as HugeRTE with community support.
So, if you are fine with what Tiny 6 provided as features, Huge RTE is the editor to pick.
If you want to use Tiny 7+ in a closed source project, you’ll need some kind of commercial license from them.
As said by @Stefan.27. As a sidenote, the TinyMCE Vaadin add-on has NOT upgraded to TinyMCE 7, so it still uses liberal license. But also for other reasons but license, I urge to go with HugeRTE addon now (Stefan has made a ton of enhancments for it that are missing from the TinyMCE variant 
).