Not sure if this counts fully as 'distributed' here, but we (Authentik Security) help many companies self-host authentik multi-region or in (private cloud + on-prem) to allow for quick IAM failover and more reliability than IAMaaS.
There's also "identity orchestration" tools like Strata that let you use multiple IdPs in multiple clouds, but then your new weakest link is the orchestration platform.
There's also "identity orchestration" tools like Strata that let you use multiple IdPs in multiple clouds, but then your new weakest link is the orchestration platform.