Please use with caution!

Finding lines containing the keyword:

grep -rs keyword

-r - recursively

-s - ignore error messages

Search is case-sensitive - use -i to deactivate

Deleting lines in all files containing a certain keyword recursively:

find . -type f -print0 | xargs -0 sed -i /keyword/d

it is case-sensitive!

Bonus: if you only want to delete the keyword and not the whole lines:

find . -type f -print0 | xargs -0 sed -i s/keyword//g

Bonus: rename only files in the current directory use the -maxdepth 1 option like find . -maxdepth 1 -type f [...].