Does this mean that if the interrogator is not Mode S, the reply
will be downgraded to Mode C? From the book of Manuals, chapter 2,
verse 2.1:

The TT21 transponder responds to both legacy Mode A/C interrogations
and to Mode S interrogations from both ground radar and airborne
collision avoidance systems. In all cases, the interrogations are
received by the transponder on 1030MHz, and replies are transmitted on
1090MHz.

The idea that the unit broadcasts more information and the fact that
many of us fly at higher altitudes than the TT21 is rated for has
caused me to order a TT22. The transponders I've used in the past had
no way to tell ATC what they were, so that wasn't a problem. I don't
care about the N Number being broadcast (when you first call ATC, they
have your number to associate with your Mode C discrete squawk code)
for any other reason than it could - in future - be a problem using
equipment outside it's certified environment. Easier to pay a couple
of hundred more thingies now and have it over with.
The TT22 and certainly TT21 still use less power than, for example,
a Becker 4401-175.
Regarding the price of Mode S transponder certification, in recent
experience it's less than recertification of the pilot!
Jim