1 foot less than 1 foot is 0 feet. It would not make you invisible.
A character that managed to do this would just look flat.
When attempting to Hide your DM gets to decide what is a valid hiding spot. I wouldn’t read Disguise Self as doing anything other than “seeming” to modify your height, so it wouldn’t help you hide. With Alter Self you could become flat, which may help you hide somewhere you otherwise couldn’t. Similarly you may be able to be blocked by cover more easily. Other than that, there is no benefit in combat.
Socially you would draw a lot of attention for your strange appearance. Again, this would be up to the DM whether you are regarded as a curiosity or an abomination.