I DID IT. I found a way for your unlocked iPhone 11 to work with Boost.
(Quick disclaimer: I spent many hours with customer support as well as Apple support and as a person who kinda works in this industry, the idea popped into my head. At the time of writing this, my phone (11 pro) is able to make calls and the 4G LTE is working like a freakin' charm.)
Now, I purchased the 4-in-1 sim kit from boost, but you can get a free sprint sim card from Apple. Put the new sim (Part number: SIMOLW516TQ if you purchased the Boost sim kit) BEFORE you turn your new phone on. Call Boost Customer Service on a secondary phone (like a household phone) and tell them that you bought your new phone with a Sprint sim inside! They will ask for your IMEI and ICCID on your new device and to tell you to turn your current phone off so they can transfer the service to your new device. That's it.
I hope this helps.
I tried this method, and it unfortunately didn't work. Boost led me on a wild goose chase. As seen in this discussion forum, a Boost Rep said sprint unlocked phones could be activated. I messaged them and I received a different message from a different rep stating that they do not activate unlocked sprint iPhone 11's, Boost could only activate unlocked iPhone 11's from the Apple store. I called Boost mobile with this information and they said they were not unlocking iPhone 11's whatsoever. I decided to do the text Boost method, and they only mentioned to check out the compatibility on their website or go to a Boost mobile store. I went to the Boost mobile store and voila, they said they could activate BUT I HAD TO PAY $35, regardless that I had the appropriate SIM card for the iPhone 11 that was sent from Boost mobile to my residence. They said I had to buy the SIM card from their store in order for my iPhone 11 to be activated ($10) and $25 for activation. So there you go. Go to a Boost mobile store, they will charge you, but they will activate it.