Change to Pandoc 2.7.2 ConTeXt template to produce PDF/A-2a.

pdfa2a.diff

*** default.context	2019-06-22 08:40:19.015364224 +0200
--- default-pdfa2a.context	2019-06-22 08:43:07.763916183 +0200
***************
*** 27,33 ****
  % make chapter, section bookmarks visible when opening document
  \placebookmarks[chapter, section, subsection, subsubsection, subsubsubsection, subsubsubsubsection][chapter, section]
  \setupinteractionscreen[option=bookmark]
- \setuptagging[state=start]
  
  $if(papersize)$
  \setuppapersize[$for(papersize)$$papersize$$sep$,$endfor$]
--- 27,32 ----
***************
*** 41,51 ****
  $if(pdfa)$
  % attempt to generate PDF/A
  \setupbackend
!   [format=PDF/A-1b:2005,
     intent=sRGB IEC61966-2.1,
     profile=sRGB.icc]
  $endif$
  
  % use microtypography
  \definefontfeature[default][default][script=latn, protrusion=quality, expansion=quality, itlc=yes, textitalics=yes, onum=yes, pnum=yes]
  \definefontfeature[smallcaps][script=latn, protrusion=quality, expansion=quality, smcp=yes, onum=yes, pnum=yes]
--- 40,53 ----
  $if(pdfa)$
  % attempt to generate PDF/A
  \setupbackend
!   [format=PDF/A-2a,
     intent=sRGB IEC61966-2.1,
     profile=sRGB.icc]
  $endif$
  
+ \setupbackend[export=yes]
+ \setupstructure[state=start,method=auto]
+ 
  % use microtypography
  \definefontfeature[default][default][script=latn, protrusion=quality, expansion=quality, itlc=yes, textitalics=yes, onum=yes, pnum=yes]
  \definefontfeature[smallcaps][script=latn, protrusion=quality, expansion=quality, smcp=yes, onum=yes, pnum=yes]