Yes a 6 year old can still use a plastic training potty.
My 6 year old has a plastic training potty that I keep near his bed so that he can use the training potty to pee or poop in if needed instead of having to leave the bedroom and walk down the hall to the bathroom.
He also uses a plastic training potty when in our van away from home when there's not a toilet nearby or he cannot hold it until we reach a toilet.
Such as on road trips etc and I've even had to use the training potty myself by squatting over it and even his older siblings use it too on road trips.
We pull over somewhere and if we need to use the potty including the 6 year old then we use the training potty.
Using a training potty is just like using another portable potty.
As long as the child wants to use the training potty then they can do so.