This was an issue back in the day; I seem to remember complaints of negative mouse acceleration and other issues with KBM in general. Has this been addressed for the GOG release?

I usually look on pcgamingwiki for such things.
https:// www.pcgamingwiki.com/wiki/Metal_Gear_Rising:_Revengeance#Input

Probably not, most GOG releases are the same as Steam releases sans a few special standout ones like RE or Alpha Protocol. But if it turns out I'm wrong, cool :)

Under "Remapping" it says "Certain keys such as A and D cannot be changed". Does that mean I can't map strafing (and forward/backward) to the arrow keys?

This is not a game designed for kb+m. It just isn't. In much the same way that people don't complain that CRPGs ported to consoles play poorly on a gamepad, you shouldn't expect this to play nicely with a kb+m setup. Just buy a gamepad.

I have a gamepad homey, but prefer playing with mouse and keyboard. Which is completely reasonable given the fact that this game was released on the PC.

It is in no way asking too much to have good KBM support - it’s not like this game was designed solely for controller because it HAS mouse and keyboard support. However, it could be improved.

I have 3 controllers for my PC alone but I also have OCD and controllers trigger me at the moment so I'd rather play with M+KB....

Handles just like it did back in 2014.
Not great, but not awful either and definitely something you need to get used to (again), especially in regards to getting the blocks and parries right which, at first can be a little tricky to pull off consistently with the mouse (and moving the camera around to face opponents for the block or parry accordingly).

Again, keyboard and mouse are serviceable and a viable alternative, but you need to put in some work to master it.

"controllers trigger me..."

Sorry, I can't tell if you are serious or if that is some kind of joke (wordplay)...

Anyway yeah I tend to agree with that other guy that nowadays PC gamers should just accept that certain kind of "console games" just play much better with a gamepad, and trying to port their controls to keyboard+mouse can in many cases be as futile as playing a PC RTS or FPS with a gamepad, an abhorrent idea...

I am currently playing e.g. Legacy of Kain: Defiance with a gamepad, I wouldn't even want to try how it plays with a gamepad + mouse (which it does support). I just don't believe it would be anywhere near as playable as with a gamepad.

That is also the reason I dislike console FPS games, using a gamepad with them just makes me mad... I originally started playing the XBox version of Halo 2 with the the XBox gamepad, it just frustrated me to no end so I switched to the PC version of Halo 2 and felt so relieved being able to play it with a mouse+keyboard. I finished the PC version instead (and I actually enjoyed the game, and the PC version had sharper/better graphics too).

I didn't even realize the pun here. I'm dead serious, I got Tourette's/OCD when I was 8 years old. Different types of items or situations can trigger heavy tics that can even lead to injuries to myself.

But I don't expect you to know anything about that....