Restatements are not required. Taking an unnecessary risk is not prohibited. Here is my understanding:
1. If you want to maintain an ongoing pre-approved plan you must restate by the end of the IRS restatement window to maintain document compliance (e.g. 4/30/2020 for a pre-approved DB plan).
2. If your plan's official date of termination is on or before the end of the restatement window, you do not have to restate. Be sure to get everyone paid out within 12 months as the IRS may consider the plan to be ongoing otherwise, and you'd have missed a restatement deadline. Any amendments that were not covered by the plan's most recent Opinion/Advisory/Determination Letter are at risk - they have not been approved by the IRS. The plan sponsor should consider that risk and weigh the cost/benefit of restating.
3. If you have an individually designed plan and you are eligible to submit your plan to the IRS for a Form 5300 determination letter, not the initial one, but one of the "other business reasons" that the IRS has yet to define, then you likely need to restate.
Lastly, if you are terminating the plan before the end of the restatement window and you intend to file a Form 5310, no restatement is required, but be sure to submit each amendment that was not covered by the plan's last Opinion/Advisory/Determination Letter (whichever applies).
Edited to add: And yes, when a plan terminates, it's written terms must be up-to-date for any changes in laws/regs/etc. to be compliant. Also, if the plan is submitting Form 5310, the document does not have to be updated by its date of plan termination like other plans. Instead, the IRS agent reviewing the Form 5310 will explain what language is missing/incorrect, ask for that proposed language, and the plan sponsor can adopt it within 91 days after the date on the IRS Determination Letter (the 401(b) period).