My inclination is to get rid of the feature for pretty much the reason you mentioned.
In any case, is the point to avoid notices getting added to HTML for wikis with certain extensions? I don't know why a production site would be set to spew notices. Either error log settings or $wgDevelopmentWarnings can handle this. If it's to avoid them in the log, again, $wgDevelopmentWarnings works.
IMO, the notices are most useful for core & extension developers testing their code (who deliberately let all warnings get spewed out). If the dev has time to work other extensions, and has the affected one enabled on the same test wikis that have other extensions being worked on, *then* it might be useful to hide certain warnings. However, it seems better to just delay the deprecation in core a cycle. The use case for the new global just seems too marginal and it seems pretty awkward and hacky.
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