A incredibly messy but reasonably effective method is a wire brush drill bit to abrade the bricks clean again.
Very very messy, as in worth building a Covid-secure plastic tent around the fireplace, and using similarly-specced breathing equipment while in there doing it, if you go down that route.
I have one very dirty/sooty fireplace from an open fire (since removed).
How can I clean this shit off? I’ve used a load of household “tinctures” with limited success as you can see.
Any ideas?