I deleted a formula which I don't want to write again, is there no way to undo the deletion? Control+z is greyed out.
If you are using ALM then you should be able to look in you Production model to find the formula that you need and copy and paste it back, assuming it did not change since your last sync.
Other than that I think your only options are to re-write it or roll the model back to before you removed the formula. (Restore information on this page: https://help.anaplan.com/anapedia/Content/Modeling/Manage_Models/History.html)
you can use the rollback (I think that’s what it’s called) from the history setting.