EW should send thread-aware replies even if threading is disabled in setings/labs.

[ara4n]

MSC3440 says in https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3440-threading-via-relations.md#sending-mthread-before-fully-implementing-threads:

Thread-ready clients should always include an m.in_reply_to property when sending
a threaded event. Unless the user is explicitly replying to another event (see "Rich replies in a thread", above),
the m.in_reply_to property should reference the latest message-like event in the
thread, and clients should also specify that m.in_reply_to
is a fallback mechanism (rather than a genuine reply) by setting the is_falling_back property to true.

However, EW doesn't do this when you reply to a threaded message (when threads are disabled in settings).

This causes confusion thanks to the reply popping up in the main timeline out of the blue, for users who have threads enabled.

It's worth noting that if the threads-aware-but-disabled user then continues the conversation without using an explicit reply, their messages will inevitably appear in the main timeline to all viewers, which will appear very confusing to users with threads enabled. Likewise threads-unaware users will always have replies appear in the main timeline. There isn't any solution to this, though, short of turning on threads. (A better solution might be to just disable threads in a room if there are clients present who have threads disabled, to avoid this sort of mess).