That's because the old law had agrandfather clause, this is a special circumstance that allows for the trust as the owner to be under the old law, it will end when the beneficiary take control of the assets...
Example 1.. can you list your bump stock in your trust and still legally own it after 3/26/19-- no (especially in ca) it is now classified as a mg and must be destroyed since there has been a change in its definition thusly changing the law with no grandfather clause(I know this "definition" is being challenged right now though) --- example 2: f your state changes their law and now outlaws suppressors without a grandfather clause then you better move those suppressors out of that state, your trust wont protect you, its trustees, or beneficiaries if they stay in that state with the suppressor
--- same goes for anything else-- your trust is still bound by all current laws, if the current law has a grandfather clause then it works for private party and trust. New law regarding suppressors says that all transfers must go through a background check (even though trust grantors, trustees, and beneficiaries did not have to for a trust started before July 13, 2016) -- so when you die and the suppressor is distributed to its beneficiaries they will have to do a form 5 transfer with bg check with prints but wont have to pay the $200 tax stamp-- but you do not need a trust to transfer a suppressor to a beneficiary on a tax free form 5, just a will.
Not only can the feds change laws that can affect grantors, trustees, and beneficiaries but so can states)
Best thing to do when it comes down to getting legal help regarding guns, trusts , nfa items is to not rely on internet help -- go get a qualified attorney
A trust ends with its beneficiaries, and it can only be changed by its settlors/grantors, so once the grantors die, it will end when the beneficiaries can legally take control over its assets, they dont last forever.
Trusts offer some protection, but can be complicated and may require more bg check with fp cards than if you just did it on an individual basis, call and talk to a dedicated gun trust attorney to get the facts for your specific state