document when backwards-compatibility features are disabled by default #13810

Job	Run time
Sanity: Linux -Dusethreads	1s
Sanity: Linux -Dusethreads	10m 38s
cygwin	0s
cygwin	33m 50s
dist ${{ matrix.os.label }} ${{ matrix.perl-version }} ${{ matrix.threads.label }}	0s
ASAN	0s
Test dist/ modules on MacOS system perl	0s
Test dist/ modules on MacOS system perl	4m 51s
install	0s
install	9m 50s
linux	0s
linux i386/ubuntu	0s
Windows mingw64	0s
Windows mingw64	35m 55s
minitest	0s
minitest	3m 41s
PERL_UNICODE	0s
macOS (Monterey) 12	0s
Windows msvc142	0s
Windows msvc142	52m 58s
involved_authors_debug	0s
linux i386/ubuntu (-Dusedevel)	15m 41s
linux (-Uusethreads)	9m 37s
linux (-Duseshrplib -Dusequadmath -Dusecbacktrace -Dusethreads)	11m 12s
linux (-Duserelocatableinc)	9m 27s
linux (-Dcc='clang')	10m 6s
linux (-Dcc='g++')	9m 53s
macOS (Monterey) 12 (-Uusethreads)	12m 17s
linux (-Accflags=-DSILENT_NO_TAINT_SUPPORT)	9m 27s
macOS (Monterey) 12 (-Duseithreads -Duseshrplib)	17m 3s
linux (-Accflags=-DNO_TAINT_SUPPORT)	9m 13s
linux (-DDEBUGGING -Accflags=-DPERL_RC_STACK)	9m 56s
ASAN (-Accflags=-DPURIFY)	21m 25s
linux (-Uusedl)	9m 7s
ASAN (-DDEBUGGING)	21m 43s
ASAN (-Dusethreads)	21m 41s
dist Linux 5.38 -Dusethreads	2m 3s
dist Linux 5.38 -Uusethreads	2m 3s
PERL_UNICODE (-Uusethreads)	8m 28s
dist Linux 5.24 -Dusethreads	1m 52s
PERL_UNICODE (-Dusethreads -Accflags=-DPURIFY -Dcc='gcc -fsanitize=address' -Dld='gcc -fsanitize=...	8m 47s
dist Linux 5.24 -Uusethreads	1m 47s
dist Linux 5.10 -Dusethreads	1m 50s
dist Linux 5.10 -Uusethreads	1m 51s
dist Linux 5.8 -Dusethreads	1m 55s
dist Linux 5.8 -Uusethreads	1m 54s
	6h 22m 2s

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