Is It True That New Houses Cannot Be Built With A Doorstep?

This is indeed true in the UK.  Since 1999 all new houses, by law, have to be fitted with either a ramp or level access into the dwelling.
This legislation is contained within Part M of the Building Regulations.  Any new house built has to comply with these Regulations.
There are some instances however, where it will be impossible to fit a ramp or level access and in these circumstances, a ramp or level access can be fitted to the back door or in 'exceptional circumstances' steps can be placed at both doors.  
However, in practice this is really only possible if the house is being built on the side of a very steep hill.
The Govenment brought this in because they wanted to ensure that disabled people could visit people in their homes.  In practice it does make life easier for parents with wheelchairs (or when getting furniture into the house!)