I just read that due to a new law, beginning in 2020 opposite-sex couples no longer have to be at least age 62 to form a Registered Domestic Partnership in California.
This opens up some temporary tax planning opportunities (temporary, as in TCJA temporary). For example, the marriage penalty of the SALT limitation can be overcome, and also mortgage interest deduction limits.
Depending on how much income is community income, some opportunities might be lost since that income must still be split on the federal single-status returns; however if one RDP makes significantly more than the other, that too can be a benefit for federal tax. Seems like $3,000-$5,000 federal tax savings would be easy to achieve for high-income RDP homeowners.
And of course, there may well be non-tax related benefits to being RDPs in California. But to end the RDP is still like going through a divorce, I'm told.