If restrictions were to be applied by the Greek government, these would be installed in order to protect Greece's foreign currency reserves. Any currency restrictions would be designed to limit currency purchase/transfer by Greek individuals, companies, etc. out of the country. Any foreigner wanting to spend (foreign) money in Greece would not only be permitted to do this but it would likely be welcomed/encouraged. Thus, preventing credit and debit cards from working would be opposite to the aims of any currency restrictions - foreign cards would definitely work so long as the issuer does not restrict their use abroad.
In all cases of currency restrictions I am familiar with, (my) cards have been usable except where/when the local banking system was not developed in a way to process them. In many cases even local individuals have been able to use cards outside their own country, if perhaps within some limitations regarding individual and annual size of transactions. Cash machines generally work with cards so they would also work for foreign visitors in a country where currency restrictions apply and who want to withdraw local currency (i.e for use in the country they are visiting). Holiday makers would in effect be unaware of any restrictions unless they wanted to reconvert local currency into their (own or other) foreign currency. The restrictions are overwhelmingly likely to be on movement out of the country concerned, not into it, anything else would have a dimension to the restrictions that have nothing to do with currencies as such (i.e. political, corruption, etc. concerns).