Personally, I think you should make the trigger warning available but not intrusive so it does not spoil gameplay for those who don't need the trigger warnings. Like, if your game includes themes of abuse, it's nice to include that in the game notes for anyone who's mental health could be affected by that. But, if you were to include that info at the start of the game along with the credits, it could be considered a spoiler.

Ultimately, people are responsible for their own mental health. But, if you want to be kind, you can make information like trigger warnings readily available 

I assume the general gist is that you should in fact add them, as, in theory, if someone would be damaging their health by your work in any way, you could be made responsible. That's at least how it would work in (some) countries.

Of course, the question then is: '"At what point would it be necessary?" - which is more of a subjective matter.

First of, not all triggers are on the same level, and it is also important how 'much/hard' it actually is within your work. Saying that your work has damaged my mental health because it 'hurt my beliefs of being a bad person in any way' (just an example) will probably not cut it in court, either.

We live in times where taking responsibility, at least to some extent, is not only important but also just good behaviour. So if you think your work could harm/hurt somebody in any form or way - you should post a disclaimer. As said, you can also overdo it and disclaim everything to not step on anyone's toes, so as long as it is not in the 'could get you in trouble' category, you should do it as your own feeling tells you.

In regards of where: It should at least be somewhere where the people it could/would affect have a 'definitive' chance of seeing it. For example, if you have a download for your work, write a disclaimer into that part of your page.

You can also add disclaimers somewhere as notes into your work as stated above. However, putting it somewhere where people that need it can find it, is not an ideal option. You should not put your work's presentation over your consumers by saying "Yeah they will find it in there, probably".

While it is true that people should be responsible for their own health (by knowing about their 'issues' for example), you are then responsible to actually give them a solid chance at even learning that your work has something that they should be aware of. Depending on what your work actually contains, and to what extent, should then be an answer to 'should I put it at the very front of my work or somewhere else".

As a person who deals with CPTSD, I can tell you that triggers are an extremely personal thing, it could be something as inocuos as a word (not even a curse or a slur, a REGULAR word) or an image. Sometimes flashbacks come back due to agression but that means agression from the REAL world, not a game or any media that's stablished as fantasy. Of course, some subjects are upseting even for people without heavy traumas.
People became obsessed with them back in the day when virtue signaling in the internet started catching on, and now we see it everywhere, it comes out as infantilizing sometimes. Like, age ratings already exist.
I personally don't believe in them, if you are playing a war game, you are OBVIOUSLY going to encounter violent situations, if you are playing a horror game, of course there will be upseting stuff, certain black comedy productions contain themes that have "triggered" me but I just don't watch them, and I don't think it should be the responsibility of the creators to look after my feelings. How are they even going to know that somewhere, PinUpCitizen is having flashbacks to her ex husband? they're not omnipresent! I don't think I have ever encounter a useful trigger warning besides the jumpscare (hate those things) and bright lights ones.

Regardless, we live in a society, so you'll have to use them wether they're useful or not.

What type of trigger warnings are we talking about? Because that word now means not exactly what it should mean.

https://en.wikipedia.org/wiki/Trauma_trigger

It now means that it will offend someone, like the overyl woke or other kinds of bigotted people that  bully people into compliance. Since offending is part of what makes art art, trigger warnings are kinda dubious. Ever seen one in front of a painting or a   movie?  I remember a disclaimer in front of Dogma. Done for laughs. But otherwise? As for games, was there a trigger warning for one of the scenes in Skyrim in the intro shortly before the dragon attacked? I think not. 

Usually an age rating would suffice  to do what people think a trigger warning should do. 

That being said, we are talking indie games here, so no rating board and small amount of negative reviews can seriously harm a game. People can react quite hateful to certain things, if they come unexpected. 

So they should either expect them by the genre, theme, description or even the game tags.   If you have content that people have strong reactions to, you might wanna check if that content is typical for the type of game and decide if a warning  were a spoiler or could help prevent 1 star ratings of disgruntled players. The medical angle is a bit far fetched imho (also think about my point about trigger warnings for other content. You would need one in front of every movie if that were a thing). Unless you advertise a kids game and continue to traumatize them, nahh, as has been explained, true triggers can be anything and the other kind are not medical. Flashing lights are medical and should be warned, this stuff is real. But offending people?