I agree that the IRS has the permanency requirement and that it takes it seriously, as pointed out by Paul I. Regarding whether or not the IRS still cares, there are a lot of IRS policies and positions taken regarding qualified plans that can be fairly old and seem inapplicable, but do not fool yourself into believing that they have become obsolete and that they no longer apply. For example, there is a 1956 regulation governing the times when certain types of qualified plans can make distributions, including in-service distributions. This regulation is still frequently cited as the reason why a distribution can or cannot be made from that type of plan. See 26 CFR Section 1.401-1(b)(1)(i) - (iii).
Regarding permanency, the IRS does not regularly cite that as a policy and none of its rulings or other guidance have relied upon it. The reason is that primarily under defined benefit plans, there was more of a potential for abuse by the top echelon of companies to terminate their plans and obtain substantial fully funded pensions upon plan termination and, if the plan were fully funded, to recover the reversions. When this was a factor, even before ERISA, generally employees were not vested at all until they were at or substantially near normal retirement age. The numerous legislative and regulatory changes that have been put into place in the intervening years, such as 415 limits, compensation limits, stricter nondiscrimination regulations, benefit accrual rules, top-heavy rules, etc. have tended to greatly curb the potential for abuses, at least to the extent of calculating benefits and providing for more meaningful benefits for rank and file employees. In today's defined contribution plan environment, the potential for such substantial disparities in favor of owners and upper echelon management is substantially attenuated. So, while it is still on the books and considered by the IRS, its prominence as a substantial arrow in the IRS' quiver of controlling abuses is greatly diminished.
Responding to your question regarding the Form 5310, an employer may but is not required to file for a determination letter from the IRS stating that the plan's termination will not adversely impact its qualification. Although it is optional, it is widely considered to be prudent to apply for such a determination letter. If the employer merely adopts a resolution stating the plan is terminated, distributes all of its assets and files a final Form 5500 for its final year of operation, there is always the risk that the IRS might conduct an audit on the plan's termination.