Ok, so when the update went live- there was a new button added at the top of custom matchmaking where previously there was one button (Now 2).
One button should read- host public , the other- host private .
To my understanding the Host public option functions as though you are holding open a public matchmaking lobby, and in this instance the privacy toggle only controls whether it will also show up as a custom lobby in the custom matchmaking board or not. Regardless of whether you toggle the option to private, players will be able to fill to your public custom match from the matchmaking que.
If you want a match this is simply un-joinable by anyone through any means, you must select the 2nd option for hosting a private custom match, wherein people can only join through the custom lobby board if you switch the privacy toggle to non-private. This route will provide the same type of lobby as was made by the host custom match lobby before the addition of the new button.
Again, if you simply want to host a custom match as you did before- select host private button to create a private custom match.
If you want your match to auto-fill from matchmaking que- select host public. If you select host public, your match cannot be made fully private, but it can be hidden from the custom lobby browser by keeping the Visibility toggle set to private (Not visible in lobby browser). If you keep it set to public (Visible in lobby browser) then people will be able to join from both matchmaking & the custom lobby board simultaneously.
(Disclaimer - although the new option appears present for versus as well, it may not function. I have not been able to get people to join into my public custom match while set to not visible in the custom lobby browser. Further information needed)