AFAIK multi-path is not too common, though most people who do multipath are using MPTCP.

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.