Not totally correct. In the US you CAN fly IFR without being on an IFR
flight plan and without a clearance, so long as you do it in Class G
airspace.Both aircraft and pilot must meet IFR requirements, however.

Ivan
CFII

What aircraft IFR requirements are there for gliders?

91.205(a) does not seem to apply (only covers powered
civil aircraft with a standard category U.S.
airworthiness certificate).

91.215 seems to exempt gliders from almost all transponder
requirements even in controlled airspace.

91.205(e) seems to still apply to FL 240+ since it
just says "civil aircraft."

Don't get me wrong: Flying south in a 2-22 in solid IFR at
FL230 using only handheld for ATC and a mag compass
as an attitude indicator, might not
be a great idea.

But as long as the pilot meets 61.57(c)(2) or 61.57(d)
(more likely) it seems generally the aircraft
instrumentation requirements are legally
non-existent for glider IFR below FL240.
And the radio requirements for controlled
airspace 91.183 could be satisfied with a
handheld.

But they may be part of experimental limitations 91.319(d)(2),
or part of the IFR required equipment in the flight manual,
or flight manual limitations against IFR, I suppose...

And if one decides they want to use a transponder in a glider
IFR, it doesn't even seem to need 91.411 (about $100) since it
isn't an airplane or helicopter.
91.413 (about $50) would still apply, I believe.