[I am a US lawyer]

It's definitely not categorically malpractice to not use Word. Depends entirely on the situation. If a court demands docx and you file an rtf, or you refuse to read a client's redlines because they're in Word, sure, that could be malpractice.

The only proprietary software that I think _might_ be malpractice not to use is a legal research service with a capable citator, like Westlaw or LexisNexis. If you rely on a case that was overruled, you can't defend a malpractice suit by saying, "I didn't want to pay for the software."

I hear CourtListener (a free legal research tool) is working on a citator--hopefully it's competitive with the paid services.