I respond to InMails where it is clear the recruiter actually took the time to read my profile and had a valid reason to believe there might be a match.

Knowing now that unanswered InMails cost them money, I'm doubling down on my stance to not answer InMails from recruiters who are spamming and/or clearly didn't bother to take the time to see if I really would be a good match or if they just matched a couple of buzzwords and hit send.