You broke down a couple of the sentences from the many questions I asked, but didn't answer all of them. What about all the adjudication of the rules that could come into play in a situation like this? Were any checks made at all that might have been appropriate? Or did the DM just say the familiar was killed? Because if it's the latter, then it's no wonder it was seen as unfair.
The thing is that the familiar is a NPC. So the DM can do whatever he wants there, especially since for example the stealth check (if any, the DM can take into account passive stealth if he wants), vs. a passive perception, etc. Even if the DM is playing things fairly, there does not need to be anything visible from the player's side.
And as you point out, if fairness was the point, the DM also had at its disposal all means to explain why it was fair, maybe after the game. Instead of that, there was a massive perception of unfairness, and an overall sentiment from the players that it was the case of "I have a a kobold ambush prepared, there is no way a stupid familiar is going to spoil this".
That is a good tactic - don't position your familiar to be killed when a monster is readying for them! While you don't get the Help action (or whatever) that round, you do get to waste the monster's attack. The player is making a meaningful choice here.
And I'm all for meaningful choices too, but once more the problem is not there, it's rather in perceiving a desire from the DM to exploit every single mistake in particular in the familiar area. Why so much hate ?
By just having flyby, it doesn't mean the familiar's not at risk. Is the familiar flying to a place where they can' be attacked? Behind total cover, for example, or out of reach? If not, then fair game.
Can we assume that the player is not stupid here and has his familiar hide when it can behind cover or out of reach ? But if suddenly the adversaries starts pulling ranger weapons or a spell just to get the familiar, it's another story.
Look, again, my point is that maybe the "kill on sight" was a bit strong a statement from you, that's all. I'm absolutely prepared to trust you on playing fairly and not inventing things like the above. But as a player, I'm wary of DMs in that specific area with statements that are that strong.
Telegraphing doesn't eliminate discovery or surprise. It just mitigates the perception of a situation as being a "gotcha."
Again, not black and white here anyway.
A monster or NPC might attack a familiar for any number of reasons that make sense in context. To suggest otherwise is to admit to a failure of imagination in a game based on make-believe, which I'm sure nobody here would do.
The problem, once more, is that the "does make sense" is extremely subjective to the good faith of the DM, since he is the one who invents the motive and the means of the adversaries. Which is why, again, having a DM making such strong statements as "kill on sight" is a bit of a warning here.
Now, if it was more along the lines of "If a player puts his familiar at risk and the familiar is obviously a threat (greater than other threats around), then there is indeed a serious risk to the familiar. But can you see how different it is from a "kill on sight" ?
I don't target familiars because players are "abusing" them. (There's that word again.) I target them because they are a valid target and because presenting a player with risk and trade-offs is at the heart of giving them meaningful choices to consider. As well, taking out resources is a good way to raise the stakes and tension.
But what happens if it does not really make sense ? For example, yes, the familiar might give one advantage or disadvantage to one attack, but is that really the major threat to the adversary ? Would he waste a valuable attack on the familiar for this, or even ready an action when he's got many better things to do ? That's the problem with the "kill on sight", for me, it's not only way too strong a statement, it also smacks of DM's hate for familiars, which is something that I think a lot of us have encountered before.