I'll copy what I've just posted over on Flyer, having been alerted to it here!
Stealth mode / No track etc were entirely rational features for Flarm and OGN to introduce, and led to a more rapid take up of both systems - which is a good thing.
Stealth mode meant that competitive glider pilots (and gliding is a competitive sport at times, remember) could use Flarm without worrying about the competitive impact. In fact, the benefits of not using stealth mode (such as spectators being able to see what's happening in competitions, and take off and landing logging) mean that it's very rarely used. So, basically, don't worry about it.
No-track was introduced by Flarm and OGN after OGN started, because there were some serious privacy issues - particularly in Germany where people are more sensitive about that sort of thing than in the UK. The feature made the problem go away, but again, I don't think it's used very much.
AIUI in any case, OGNR will rebroadcast stealth mode gliders. Perhaps not no-track ones, whose packets probably get dropped in the OGN receiver.
As to 7000 Flarms in the UK - I don't believe it. It's possible that 7000 have been sold to the UK over the history of the product, I suppose (though the propensity of glider pilots not to throw stuff away makes this unlikely in my view). But, there are (o) 2000-2500 gliders in the UK (2310 according to wikipedia), and as other people have pointed out, the penetration of Flarm into other sectors in the UK is low. Anyone who keeps an eye on the OGN plots knows this. I'd have thought 2000-2500 active Flarms is much closer to the truth. [so same order of magnitude as PAW, and currently a lot more than ADSB in GA aircraft].
Paul