Last I checked, It's part of LTE standard, but used exclusively in South Korea. Some providers across the world provide multi-path gateways: in France, OVH has OverTheBox which is line-provider agnostic, Free has ADSL+4G aggregation where fiber isn't available. In Switzerland, Swisscom also has some multi-path offer.
Also, Apple's Siri uses MPTCP. (And developers love the fact that only Siri can use it, and not other apps).
Google/Android said no to MPTCP, saying QUIC/HTTP3 already has the provisions needed to do multipath, but I don't know if Google actually uses it somewhere.
MPTCP's design was fundamentally incompatible with QUIC's UDP transport and session model.
From what I understand, neither MPTCP nor MPQUIC are designed to share multipath routing via a gateway (e.g. a bunch of devices sharing a multipath aggregation of 3 4G modems). It's all designed for a single device with multiple interfaces.
Looks like there’s been an API for at least a few years. http://blog.multipath-tcp.org/blog/html/2017/07/05/mptcp_exp...
https://estore.peplink.com/categories/speedfusion-cloud/spee...
We use to use Peplink at a company I worked at
Maybe they looked at their numbers amd couldn't didn't see the free tier paying for itself in conversions, but it did convert me.