What's your point here, exactly? That they should, or should not be able to do that? Because this already happens, except it's even worse than letting people do it, because Twitter does it by itself. With zero oversight by anyone (not that other big tech platforms don't do the same). Twitter already rank boosts content it likes and content it doesn't like gets downranked, shadow banned or just permabanned. It already filters trends and search and autocomplete. So we have a situation where a big tech company does what it wants and does it so much that it literally skews elections (all of this is common sense, but there are also many peer reviewed studies about this, some authors of which have testified before congress and some of which are represent bases for many US & EU investigations into the big tech monopolies/cartels). So given all this, is it really a problem to let people downvote stuff... or is it actually about not letting the people you don't like have a vote?
Youtube dislikes were completely fine 99.999999% of the time and lasted for like a decade without issues. Until someone like you, sadly in a position of power, decided that (speculation starts) Biden got too many dislikes on his ramblings and it was a bad look for Google (speculation ends).
Some people just HATE democracy because democracy means even dumb people, rednecks, racists, sexists and all kinds of "irresponsible" people get to vote. Ironically, most of the time (in recent history at least), these people who hate democracy call themselves democrats or liberals. In reality, they're anything but that. Actual liberals recognize that everyone deserves a to have a voice in a democracy. Even the bad voices are valuable, because they allow the marketplace of ideas actually do its magic.