Revert "Deprecate key="a,b" and by="a,b""

[MichaelChirico]

This is a placeholder follow-up to #6047 (which closes #4357). What to do with this PR depends on the outcome of the revdeps checks resulting from #6047. This needs to be decided before 1.16.0 release.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.53%. Comparing base (a11a2e2) to head (cb9ad20).
Report is 8 commits behind head on master.

❗ Current head cb9ad20 differs from pull request most recent head a55ff7c. Consider uploading reports for the commit a55ff7c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6049   +/-   ##
=======================================
  Coverage   97.52%   97.53%           
=======================================
  Files          80       80           
  Lines       14978    14916   -62     
=======================================
- Hits        14607    14548   -59     
+ Misses        371      368    -3     

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[tdhock]

there are 11 revdeps that fail because of #6047
zeitgebr
tidyfst
tidyfast
pointdensityP
maditr
ggPMX
expss
damr
calmr
DramaAnalysis
BTYDplus

[MichaelChirico]

Just filed fix PRs with all the downstreams that host their code on GitHub (all but {pointdensityP}) for which a simple regex search identified some issues. More might remain e.g. in cases like key="a,b,c"; data.table(key = key) which are harder to identify.