Being short doesn't make you automatically undesirable the way that being fat, or in a wheelchair, or missing a limb, or being darker skin or having a a specific occupation is.
You can check my other posts in the thread if you want to know my position on the topic, but right now, I disagree with the quoted. If we're talking about dating (and it seems you are based on the rest of your post), I don't think there's a single trait (physical or otherwise) that will make someone 100% un-dateable. There are traits that, depending on the cultural/societal norms of the country you inhabit, will get you "filtered" by a certain number of people, automatically, no questions asked. Is it a majority of people? I don't know (probably not).
With that said, height is definitely one of those traits. If by "automatically undesirable" you actually meant "universally undesirable" then neither of the other traits you mentioned apply either. If by "automatically undesirable" you meant "automatically undesirable to a certain number of people" then I also disagree, since short height (just as the other traits) is most definitely a filter for some people. I believe that those people have the right to that preference; dating is a privilege, not a right, so short people (that includes me btw) are not entitled to being given a chance or whatever.
I don't mind being filtered because of my height when it comes to dating. I have preferences myself, and wouldn't date someone outside of my preferences, whether they are considered shallow or not by others, it's my personal taste when it comes to looking for a partner or someone fun to spend time with, etc. The whole "manlet" thing actually makes me laugh, like, I can't believe people use that term. I would definitely be insulted if someone were to make fun of my height in a professional or social setting, but that's mainly because I'm 40 now and I expect people to behave like adults, not like children.
People on Twitter are being assholes, that's nothing new. Definitely not "harassment", and definitely not comparable to racism or other types of prejudice, in my opinion.
Nah. Not quite, not by a long shot.
Surely if you are shorter than average you can just not mention it?
In my country, where I'm just below average, I don't mention it in my profile, but people still ask. Some have mentioned it's a deal breaker, so I say my goodbyes and that's it. Some have mentioned it's ok, and so the conversation continues. Some haven't even asked.
In a country like America where I would be several inches below not only average but what's considered desirable, I would most definitely include it in my profile, so only people that don't care about that would "swipe right" or whatever.