The two collaborated on the 2015 song "Pretty Girls." The song didn't perform super well, and Iggy tweeted at the time, “It’s difficult to send a song up the charts without additional promo and TV performances, etc." — implying that Britney's team had been unwilling to do promo. The media played this up as a feud, with Iggy getting a bunch of backlash.
The following year, Iggy then recalled how Britney's team searched her home before they were allowed to work together: "They came and checked my place to make sure I wasn't trying to stash anything weird and, like, get it to her or something...to make sure I wasn't a bad influence. And I'm not. I passed the test!"
Fast-forward to now, when one Twitter user called out Iggy and other Britney collaborators for not speaking publicly about Britney's conservatorship.
However, Iggy herself hopped on the thread to remind people what she had previously said...
...and because of her NDA, we're unlikely to hear more about what happened.
Iggy then explained that she had reached out to Britney privately:
And she reiterated that she "actually cares":
You can read more about what Britney said in court here.