A user on the Debian forums emailed and got a reply from one of the developers of the KPTI patch in the kernel:
> I'm writing to you because I noticed your involement with the KPTI/KAISER
> patches. Across several varieties of linux distributions, users have
> noticed that kpti is impossible to enable because it depends on x86_64.
> Many of us are concerned that we are running 32-bit systems that are
> still vulnerable to meltdown; we are also concerned because it's a
> handful of users who have brought this to light, and major news and
> information from our distros are keeping silent on the topic. We are all
> wondering if you could shed some light: in particular, is x86 vulnerable?
Yes, 32bit is vulnerable. We haven't yet had time to look into that as the
vast majority of systems, especially the most endangered cloud stuff, runs
64bit. We know about it and the 32bit mitigation has been under discussion
already, but I can't tell at the moment when we are going to have that.
Sorry that I can't tell you better news.
So that's the situation now.