James Salsman wrote:

Are there any disadvantages to a warrant canary which would outweigh the corresponding expected increase in improvements from anonymous editors?

This question is presented in such a way that it's difficult to answer, in my opinion. You'll need to provide additional context.

Related to your question, below are some data points that may be helpful to you or others.

In 2013, the Wikimedia Foundation said:

--- The Wikimedia Foundation has not received requests or legal orders to participate in PRISM, to comply with the Foreign Intelligence Surveillance Act (FISA), or to participate in or facilitate any secret intelligence surveillance program. ---

https://blog.wikimedia.org/2013/06/14/prism-surveillance-wikimedia/

There's also this note from Luis Villa also from 2013:

--- 1) We've flat-out denied any sort of involvement in this, and we continue to stand by that denial: https://blog.wikimedia.org/2013/06/14/prism-surveillance-wikimedia/

2) Take with a grain of salt, of course, but our understanding (based on the few gag orders that have been made public) is that we could be forced to not confirm having received a National Security Letter, but we can't actually be forced to lie about it. In other words, if we'd received one we would not be allowed to say "we've received one", but we also could not be forced to deny it - we'd always have the option to remain silent instead. ---

https://lists.wikimedia.org/pipermail/wikimedia-l/2013-August/127380.html

I believe there have been similar statements made subsequent to 2013, but I don't know if we have them indexed somewhere. We probably should.

There's also this reply from 2016:

--- Yeah, sorry about that. I am not subject to a National Security Letter.--Jimbo Wales (talk) 15:55, 8 January 2016 (UTC) ---

https://en.wikipedia.org/wiki/User_talk:Jimbo_Wales/Archive_199

MZMcBride