Two significant election law changes made some time ago have just come to my awareness…South Dakota banned fusion in 1999, and no one who cares about this seems to have known about that. Also, in 2001, Vermont lowered the petition requirement for independent candidates for all statewide office (except president) from 1,000 signatures, to 250 signatures. It is not known why this happened.
I think you have the wrong information. It used to be 500 signatures, not 1000, to run for U.S. Senate and U.S. House of Reps. My understanding is that
this was done for the benefit of Bernie Sanders who
is running as an independent.
Another little tidbit about Vermont ballot petitions.
NONE of the signatures are verified. Only the names on the petitions are checked to see if they are registered voters and only for independent candidates, not primary ( party ) candidates.
How can you have a valid signature on a petition if the signature is not checked against the file copy? All they check are the names.