Oh hey they added KBM support after all? Cool, that's what I was waiting for!
But hmm... these steam reviews are pretty bad. Let's take a look ...

Unironically, the KBM keybinds for MGS on Steam are:

• Switch Weapon: 0
• Switch Item: 9
• Shoot: U (??)
• Melee: H (??)
• Hide Weapon: M (??)
• Codec: ESC
• Pause: TAB
• First Person: F
• Crawl: Space
• Menu: LITERALLY NOBODY KNOWS YET LMAO

And no you cannot rebind them.

Locked at 720p.
"Mouse support" is just emulating the right analogue stick, so there's no mouse aiming (can't aim with the right stick in MGS2).
Keyboard controls are nonsensical and can't be edited.

Ah.