I think I know what's going on: iputils[1] ping can't ping IPv6-mapped IPv4 addresses, but inetutils[2] ping can. And look inside the parens:
$ ping ::ffff:127.0.0.1
PING ::ffff:127.0.0.1 (::ffff:127.0.0.1) 56 data bytes
says the non-working one,
$ ping ::ffff:127.0.0.1
PING ::ffff:127.0.0.1 (127.0.0.1): 56 data bytes
says the working one. In Wireshark, the latter appears as ICMPv4 packets on the lo interface, whereas the former does not appear at all(?..). So overall this makes some amount of sense: you can write a TCP-using program that's agnostic to whether it's running on top IPv4 or IPv6, but you have to use different ICMP versions for IPv4 and IPv6. I actually don't know why it has to be that way.
(My initial confusion was because I thought 'o11c was saying they could ping ::ffff:127.0.0.1 but not .2. It makes much more sense for either both or neither to be pingable.)
The good thing is that verifying the other contact is invisible to the server in Signal. This means that it's stochastically sufficient that a few people do check their contacts in order to see whether there is any widespread MITMing going on.
It's less encrypted. E.g. you'd think that emoji reactions are end-to-end-encrypted (as they are in Signal). But they aren't[1]. I expect similar implementation issues wrt. the encryption in Matrix.
Some people get very emotional about the games that they play and will pay to have them DDoSed because of something or someone they're angry about. Others just love to cause chaos and will happily buy a DDoS attack to screw other people over. They even get to watch the outcome in real-time because of streamers.
The Digitale-Dienste-Gesetz (https://www.gesetze-im-internet.de/ddg/__5.html) in § 5 imposes this requirement on "geschäftsmäßige, in der Regel gegen Entgelt angebotene digitale Dienste", i.e. "services operated in a business-like manner, usually against payment". The key word here is "geschäftsmäßig" (literally translated "in the manner of a business", "business-like"), which within German law is held to be a weaker requirement than "gewerbsmäßig", that is "commercial".
"Geschäftsmäßig" can be anything you keep on doing regularly, independently of any profit motives – you can find a few discussions of that online, and amongst other things, the same term was also used in now-overturned legislation against commercial assisted-suicide, where it caused the same interpretation problems whether doctors or non-profit associations nonetheless might be construed to be acting "geschäftsmäßig" even if they didn't take any payments.
The "usually against payment" bit means that anything done for money can unambiguously by default be presumed to be business-like (and even worse, one possible additional interpretation is that anything other people usually do for money can be argued to be "geschäftsmäßig", too, even if you yourself offer it for free), but the reverse isn't necessarily true. Keeping with the F-Droid sphere for example, something like https://android.izzysoft.de/ I think could (even if you subtracted the few affiliate ads) very well be argued to be operated in a business-like manner, since it's relatively big and professionally-done enough.
The other relevant law is the Medienstaatsvertrag (https://www.die-medienanstalten.de/fileadmin/user_upload/Rec...), where an exception from the imprint requirements in § 18 is made only if operated for "persönlichen oder familiären Zwecken", i.e. for "personal or family purposes". (And journalistic offerings, which can potentially cover some blogs or forums for example, are even subject to extended imprint requirements.)
The "family" bit is relatively clear, but "personal" purposes are once again more interpretable. A wide reading of "personal" would indeed cover all sorts of non-commercial hobby pages, though in that case you could ask yourself the question why a separate exception for family purposes would then still be required, too?
A narrower reading of "personal" on the other hand would only cover stuff that's literally only intended for you, like login pages to your personal webmail or Nextcloud instances, your private picture or file hosting and similar things. Some lawyers even go as far as including a private diary (that for some reason however you decide to publish online and even without password protection) within the category of "personal" things (though depending how much your diary refers to identifiable outside entities, it could also be construed to be a journalistic offering and therefore definitely subject to imprint requirements), but that's about the limit.
Hobby pages intended for the non-specific general public (outside of just family and friends) are therefore very much in a grey area as to when exactly the imprint requirements are starting to become applicable even if you don't do any ads at all.
