That's a great question! To clarify, deleting Envelopes will not delete the associated transactions. Instead, those transactions become 'unassigned', meaning that they no longer belong to an Envelope. At that point, you could reassign them to a new Envelope, or leave them as is. If you do leave them unassigned, know that those transactions will no longer show up in your spending reports.
Alternatively, you could create an 'Archived' Envelope Group. This is my preferred method, because I can keep all of the Envelopes and the transactions remained assigned to the correct ones. With this method, you could edit the Envelope and give it a budgeted amount of 0.00, so that it's no longer calculated into your Monthly budget, and you'll discontinue filling it.
To create an Envelope Group on the website, you'll format the Envelope's name with a colon, so that it looks like this,
Archived: Car Payment Archived: Gym Membership etc.
Does this make sense? Let me know if you have more questions.