type t.txt
this is a line
#this is a commented line
;is this a commented line
last line

for /f "usebackq tokens=* eol=; eol=#" %f in (`type t.txt`) do (echo "%f")
"this is a line"
";is this a commented line"
"last line"

for /f "usebackq tokens=* eol=;#" %f in (`type t.txt`) do (echo "%f")
#" was unexpected at this time.

for /f "usebackq eol=; delims=" %%f in (`type t.txt`) do (
    for /f "eol=# delims=" %%Z in ("%%~f")  do echo %%Z
)

for /f  "usebackq delims=" %%f in (`type t.txt^| findstr /i /b /v "; #"`) do ( echo %%f )

for /f  "usebackq delims=" %f in (`type type.txt^| findstr /i /b /v "; #"`) do ( echo %f )

findstr /V /B "# ;" t.txt