Why isn't clearfield suppress issue number in footnotes in Biblatex?

AtEveryBibitem applies only to the bibliography. If you need your clearfields to apply to citations as well, you can use AtEveryCitekey. But of course that doubles the lines of code you have to write.

documentclass{article}
usepackage[british]{babel}
usepackage[autostyle]{csquotes}
usepackage[backend=biber,
  style=oscola,
  giveninits,
  ibidtracker=true, ibidstyle=lc,
  ecli=true,
]{biblatex}

AtEveryBibitem{%
  ifentrytype{article}
    {clearfield{issue}%
     clearfield{number}}
    {}}
AtEveryCitekey{%
  ifentrytype{article}
    {clearfield{issue}%
     clearfield{number}}
    {}}

begin{filecontents}{jobname.bib}
@article{VanderMeulen2013,
  author  = {van der Meulen, Bernd},
  doi     = {10.3390/laws2020069},
  issn    = {2075-471X},
  journal = {Laws},
  number  = {2},
  pages   = {69--98},
  title   = {The Structure of {European} Food Law},
  volume  = {2},
  year    = {2013},
}
end{filecontents}
addbibresource{jobname.bib}

begin{document}
  A sentence.autocites{VanderMeulen2013}
    
  printbibliography
end{document}

---

In order to completely remove fields from the output I usually prefer a sourcemap solution that deletes the fields when they are read from the .bib file.

documentclass{article}
usepackage[british]{babel}
usepackage[autostyle]{csquotes}
usepackage[backend=biber,
  style=oscola,
  giveninits,
  ibidtracker=true, ibidstyle=lc,
  ecli=true,
]{biblatex}

DeclareSourcemap{
  maps[datatype=bibtex]{
    map{
      pertype{article}
      step[fieldset=number, null]
      step[fieldset=issue, null]
    }
  }
}

begin{filecontents}{jobname.bib}
@article{VanderMeulen2013,
  author  = {van der Meulen, Bernd},
  doi     = {10.3390/laws2020069},
  issn    = {2075-471X},
  journal = {Laws},
  number  = {2},
  pages   = {69--98},
  title   = {The Structure of {European} Food Law},
  volume  = {2},
  year    = {2013},
}
end{filecontents}
addbibresource{jobname.bib}

begin{document}
  A sentence.autocites{VanderMeulen2013}
    
  printbibliography
end{document}