Kastelo Inc.
The company behind commercial support for syncthing. syncthing.net names it directly: Kastelo
"provides commercial support for Syncthing and sponsors Syncthing with development resources," and
links to kastelo.net/stes/ for the support product. Two other sponsors are named beside it —
Möbius Sync (the iOS client) and REEF Solutions.
Why this spoke pages it. The recurring question here is who is paid to notice when it breaks. ever-gauzy's answer was nobody, and the price was that nobody is paid to notice either. Syncthing's answer is a company selling support around software it does not own — the project stays MPL-2.0 and community-governed, and the commercial layer sits outside it. That is a distinct arrangement from a vendor open-sourcing its own product, and this spoke had no example of it.
Thin node: paged from Syncthing's front page and the company's own title tag, both checked. Nothing here about its size, its customers, or how much of Syncthing's development it actually funds — the sponsorship is stated by the sponsored project and unquantified.