I checked the Core Lightning github and multipath payments are "on" by default there too. See, for example, this issue:

GitHub

MPP pay splits into too many subpayments · Issue #3926 · ElementsProject/lightning

I have an outbound channel with an BTC to Lightning exchange worth 2.000.000 satoshi, with enough capacity. I try to swap my 2.000.000 satoshi for ...

It involved an error report where a user encountered this entry too many times in their error logs: "Detected large payment, splitting into 100 sub-payments" I checked the Eclair github too and multipath payments are "on" by default whenever trampoline routing is used (which is the case in Phoenix, which always uses Acinq as a trampoline node)

GitHub

eclair/eclair-core/src/main/scala/fr/acinq/eclair/payment/send/TrampolinePaymentLifecycle.scala at 826284cb277c28c7eef14aa275f3d6e3255c8e66 · ACINQ/eclair

A scala implementation of the Lightning Network. Contribute to ACINQ/eclair development by creating an account on GitHub.