Perhaps it’s just me, but it has happened repeatedly that I hit delete button when i really wanted to open Layer Options.
Suggestion:
If you can, I’d recommend learning the key commands for the most frequently-used options dialogs: Ctrl/Cmd-Shift-L for Layout Options, Ctrl/Cmd-Shift-E for Engraving Options, and Ctrl/Cmd-Shift-N for Notation Options.
The Delete button is always at the right edge of action bars, and is usually separated from other options.
If you accidentally delete layouts, you can quickly restore all default layouts (a separate part for every player) by choosing Setup>Create Default Part Layouts.
… or cmd/control Z (Undo) will get the Layout back.
It was disabled!
I “discovered” yet another way to delete all layouts today… While in setup mode, looking at the score, I just wanted to delete something on the score. Clicked and hit delete button, only to find “Start your project by adding some players” dialog displayed in all modes.
I did not understand what happened. My players were there. What’s happened??
Then it hit me: of course, I must had been in setup mode hitting delete? And sure, all layouts are gone. And again, “undo delete layouts” is disabled!
Improvement suggestions:
Deleting layouts is undoable. I’m not sure why you are finding that it’s not.
You can’t select anything in the music in Setup mode, so as tempting as it can be, make sure you’re in or switch to Write mode before trying to edit the music.
I don’t know why, but I do. Please see the attached video. Undo is disabled, cmd Z doesn’t work, either.
It seems that if there is only one layout and you delete it, it is not undoable. With more than one layout, it is undoable. But if it’s a Part layout, you could get it back (I think) by going to Setup (between View and Window menus) and selecting Create Default Part Layouts. You can also get Full Score or Part Layouts generated by clicking on the appropriate icons in the right hand pane. (Apologies if you already know this). 
When I click on a score in Setup and then press the DELETE key, I get a confirmation query.
Improvement suggestion?
Work more carefully and take responsibility rather than blaming the program when you make a mistake.
And Undo works just fine for me.
Derrek, I believe that message appears when you delete a Player, rather than a Layout.
Derrek, I think he is deleting the layout from the Layout menu, and there is no confirmation. I believe Undo (for some reason) is not available if there is only ONE layout and it is (mistakenly) deleted. I’ve never had this happen to me, personally.
Yes, Lillie. Thank you, that is correct.
And musicmaven, you are also correct: if one deletes all layouts to a file, undo is not available. But I still think that selecting all layouts before trying to open them would be a rare occurrence, and then clicking delete is not Dorico’s fault.
The poster in question has made some valuable suggestions but also seems on numerous occasions to criticize Dorico for not protecting him or her from odd and hasty errors. At a certain level one has to take responsibility for what one clicks. Perhaps there should be a cautionary panel like the one above if one chooses to delete all layouts, but ultimately that sort of approach to every possible error can be a hindrance rather than a help.
I agree… it does seem interesting to me that for how comprehensive the Undo is in Dorico, that we’ve found one thing it is not “undoing”! 
@Daniel, if this is the case - that deleting the last layout disables undo (same happens if you delete all layouts at once), it could be that Dorico‘s internal state is reset by this action and Dorico falsely assumes this is a new project, hence no undo history.
The screen which is displayed at that state (Start your project…) supports this hypothesis.
Yes, the specific case of deleting the last layout in a project will indeed make Undo and Redo appear as disabled; we’ll look at relaxing this in a future version.
