We did explicitly choose MIT for the freedom of end users to deploy and modify the code how they want - and tried to open source pretty much everything that doesn't have a hard 3rd party dependency. We do touch a bit on how we think about the open core model as well in the README, and largely align with Gitlab's stewardship model [1] when it comes to paid vs OSS. In this case, a contribution to add SAML specifically to OSS will likely not be merged. It'd also introduce complexities with maintaining that alongside our cloud version that already includes a specific implementation of SAML.
[1] https://handbook.gitlab.com/handbook/company/stewardship/