With 43s we can only make the lowest flush, because any hand with the 2 of that color would have a 5 or higher with it.

And furthermore, 23s, 43s and 45s are likely to make the idiotend of the straight.

So, should these hands played never voluntary?

Who told you that you should play them? Let's start there.

"Never" is a strong word. For a good enough price it will certainly be profitable to play hands like 43s. Imagine UTG min-raises and everyone calls. Would you throw away 43s in the big blind instead of calling 1 BB more into a pot of 11 BB? It's true that 43s is worse than a hand like 98s, but that's only partly due to 43s making lower flushes and straights. A bigger reason for 98s being better is that a single pair of 8's or 9's is the best hand more often than a pair of 4's and 3's.

There can be a lower flush than 43s, by the way - four to a flush on board and the opponent holds just the 2 of that suit

You should rarely be playing the lowest suited connectors, but they are viable raises/calls in some situations. FWIW, 53s is slightly preferred to 43s. They both make the same number of straights (3) using both cards, but one makes marginally better pairs (and flushes) than the other. (54s is better than both, as it can make four straights and has an even better kicker!).

There's a cool video by Annie Duke where she holds a lecture at a college and argues that suitedness is not the reason why you should only play (small) connectors if they are suited. Because if you make the flush everyone can see it on the board. If you win with it you'll not get paid much because your opponent doesn't have a flush (or an even worse one). Against a high flush you'll lose your stack: Potentially small gain but potentially big loss. The suitedness is more of an 'emergency out' in a sticky situation with a runner runner or a lucky river.

She says: The reason why she only plays suited connectors is that it prevents her from playing too many bad hands. If you play every middling and low connector (which I'm defining as like 98o or lower)... and every gapper and 2 gapper and whatnot...then you are playing waaaaay too many hands that are behind preflop.
I.e. you're opening a massively -EV range against any moderately tight range...you'd need a huge skill edge to nullify that.

I found this a very interesting take on the subject.

As for playing them at all: Sure. E.g. as a resteal. I'm personally a sucker for 52s (which I've dubbed the "spanish inquisition"...since: "Nobody expects the spanish inquisition" )

...of course one reason to occasionally play the small suited connectors is: if you never play them a perceptive opponent will instantly know that you missed a 334 flop and can bet with near perfect information