Pretty much, the most well known example of such a pre-69 trust is the series of Kennedy Trusts set up by Joeseph P Kennedy (JFK's Dad). It's what allows the various members of the Kennedy family to avoid paying most taxes and grow their wealth, despite the dilution of the family fortune throughout several generations.

But yeah, if you can get your hands on a pre-69 trust, the only thing you'll have to do is pay a one time "gift tax" which is levied at the normal income tax rates to the trust. After that, all money generated by the trust is exempt from most federal level taxes.

As for the expiration date, it depends on how it's set up. The Rockefeller trusts for example, had expiration dates because they were set up in an era where it was required for trusts to end after a certain period (usually 25 years) and then be "renewed". This is why the Rockefeller foundation was started, because it allowed Rockefeller and his family to maintain control of their assets throughout the generations under the guise of "charitable work".

By the Time the Kennedy's made their fortune, a different form of trust called a "dynasty trust" had come into play, as several states had legalized such trusts. Basically, this type of trust allows for either extremely long renewal periods (anywhere from 300-3,000 years), to actually being infinite in their renewal period. It depends on what state you set it up in (South Dakota and Delaware are the best states to set up a dynasty trust in FYI, followed by Tennessee, Nevada, and Wyoming).

If set up properly, these types of trusts will allow a "head of family" to essentially pick their heir from each generation of the family, and hand of the rights and position of "trustee" to them each generation. The Trustee then sets up who the current beneficiaries are, usually themself and their family members, and takes on the corporate position of chairman, while hiring a CEO to run the trust as an actual business. By doing it this way, each time you pass on the trust from generation to generation, the renewal period resets (not that it matters in the states that allow perpetual trust renewal periods), meaning it's easy to pass on generation wealth, and the assets held in the trust are safe from the inheritance tax, debtors, creditors, spiteful ex spouses, lawsuits, etc. etc.

When you hear about super wealthy people setting up trusts, it's usually a dynasty trust because it protects them from being sued, divorces, etc. and still allows them to pass on their wealth to their heirs without worrying about probate, inheritance taxes, etc. While allowing the family to maintain overall control of the assets. However, unless it was set up before 1969 (and thus was grandfathered in as you put it), that's where their benefits end. They still have to pay all other taxes, including federal income tax.