The classic version of Google Sites will phased out beginning in Q4 of this year. Tools for migration to the new, current version of Google Sites will be made available to users of classic Google Sites. Once the official deprecation timeline for classic Google Sites is announced, those sites will still be supported for at least a year although you won't be able to create new Sites using the classic editor.
What all this means is that if you're thinking of building a website by using Google Sites, use the new (current) version of Google Sites because eventually you're going to have to use it anyway. If you already have a site established through the classic version of Google Sites, you have some time before you have to make the switch.