Are there other usages of the expression “don’t ask don’t tell” besides the one from its Wikipedia page?
Don’t ask, don’t tell (DADT) is the
term commonly used for the policy
restricting United States military
personnel from efforts to discriminate
or harass closeted homosexual or
bisexual service members or
applicants, while barring those who
are openly gay, lesbian, or bisexual
from military service.
Or is the expression also used in contexts not related to homosexualism in the military?
No, this is the name of a specific policy regarding gay servicemen and women in the US military. It is not a general term in the military or elsewhere.
(If you see it anywhere outside of this context, it is almost certainly at least making reference to the military policy, e.g. a satirical reference.)