The term Atex is derived from the French title of the original Atex directive Appareils destinés à être utilisés en ATmosphères EXplosives (Apparatus for use in explosive atmospheres)
The original Atex directive 94/9/EC dated 23rd of March 1994 became mandatory in July 2003 and all new equipment and protective systems for use in hazardous areas have been subject to it since. At that time any pre-Atex equipment already placed on the market could be continue to be used but should have been subject to ongoing risk assessments.
The Directive was updated in line with the 'new legislative framework' and ATEX directive 2014/34/EU became mandatory in April 2016.
Atex is an EU legal framework for the use of equipment within the hazardous area not a product certification. Therefore, by definition, Atex has no validity outside the EU (including a no-deal UK) unless other regulatory measures are in place.
Obviously the Atex documentation could be accepted outside of EU as it is in some non-EU countries. Again, by definition, Brexit without a deal, will divorce the UK from the European legal framework and there are a number of measures in place to handle that including UK certification marks. In the transition period until 31st January 2020 (or earlier if a deal is agree) all Atex documentation is accepted.
See the Brexit page for more information