For the purposes of following along with the underlying forces affecting mortgage rates, Freddie Mac MBS essentially never mattered.  Fannie had always been the big show, and that message was driven home with the implementation of the Single Security Initiative (SSI).

Haven't heard of the SSI? 

Don't sweat it.  If you haven't heard about it, that means it's completely inconsequential for you.  It only matters on the secondary mortgage market and even then, it doesn't have a significant bearing on mortgage rate momentum, intraday reprice risk, etc.  For all intents and purposes, it's a housekeeping change. 

So What Is It?

How much detail do you want?  Short version: Fannie and Freddie still exist separately, but they've aligned their standards just enough to issue the same MBS product: Universal MBS (or UMBS).  To be precise, FHFA (the regulator for both Fannie and Freddie) created UMBS in Fannie Mae's image and then Freddie tweaked what it had to in order to fit that mold.

The effective result is that Freddie Mac no longer has its own, separate, differently-priced MBS coupons.  Everything is Fannie and/or UMBS now--whichever you want to call it.  Technically, it's UMBS, but it's also still FNMA MBS (and it's ALSO also FHLMC MBS that fits in the FNMA/UMBS/SSI box.  

How does this affect me?

It doesn't.  If you were ever watching Freddie MBS for some reason, unless you were in a shop that only dealt with Freddie and you wrote loans were funded and serviced by lenders that absolutely never touched Fannie MBS, you should have been watching Fannies anyway.

What should I do to account for this change as far as MBS Live is concerned?

If you have any alerts set up based on Freddie MBS, they won't work now.  Here's the link to change your alert settings.  If you're the typical originator who's interested in following reprice risk and general rate momentum, the current coupon of focus is FNMA 2.5 for 30yr fixed loans.  

Why doesn't the MBS Live Dashboard say "UMBS?"

It will, eventually.  There is a significant level of complexity involved in updating MBS live.  We've been beta testing it for months because we don't want anything to break when the entire community is migrated.  For what it's worth, professional trading/analytics terminals (such as Refinitiv Eikon) continue to use FNMA codes for their UMBS instruments.  Even then, UMBS will only represent a visual change from FNMA.  Nothing about the underlying price/data feed will have changed.

But I still use LP and get different results running loans there vs DU/Fannie.  How can that be the case if they are both the same MBS now?

UMBS doesn't require that the AUS's perfectly align, only that a specific set of guidelines is met in both cases.  If those guidelines are met, and each agency can meet them with slightly different AUS protocol, they're free to do so.  Either way, the LP loans you're writing will end up as UMBS, just like the DU loans others are writing.

So that's it?

Not by a long shot!  That was the short answer.  If you want the long answers, here are a few links:

FHFA's Original Guidance on SSI 
Freddie's SSI Page 
Freddie's SSI FAQ
Fannie's SSI Page
Fannie's SSI FAQ

MND's coverage:

Fannie and Freddie Another Step Closer to Sharing Single Security

Single Security Praised by Urban Institute

Freddie and Fannie Changes - Single Security Moving Forward

Single GSE Security Release Date Set - Mortgage News Daily

This Week Marked Low-Key Launch of the GSEs' Single MBS

MBS Pricing Snapshot
Pricing shown below is delayed, please note the timestamp at the bottom. Real time pricing is available via MBS Live.
FNMA 2.5
101-04 : +0-04
10 YR
1.3320 : -0.0450
Pricing as of 2/25/20 12:20PMEST