Not sure, but if DS9 wasn't being protected for widescreen in 1995, I'm not sure why Voyager would have been. Maybe the later seasons?
Someone could ask Rick Berman on Twitter.
My vague recollection, and I'm thinking back to the mid-90s Star Trek: The Magazine here, so I can't vouch for it's accuracy, but I do seem to recall reading that they
at least did camera tests for 16:9 of some kind, possibly of the empty sets rather than during the actual filming. But I can't really remember now. For curiosity's sake I'd love to see it confirmed or denied. Out of all the Classic TV Trek sets, Voyager's bridge set seems to have been the one that would have benefitted the most from being in widescreen, having been built quite wide and with lots of extra detail on either side of it. Nooks and crannies everywhere.
Babylon 5 and Stargate both filmed 'safe' for widescreen, even though it was years before that aspect ratio became common, which is why true widescreen versions of the episodes are available today. (The actual actors were still staged for full screen 4:3 broadcast, so whatever action takes place in the widescreen versions is kept carefully in the middle of the image.)
Widescreen versions of Buffy exist too, but the action wasn't appropriately staged for it, so there's lots of little mishaps on the widescreen episodes, like actors standing off to the side waiting for their cue, or bits of the set that weren't finished being visible on the screen. In other words, they weren't staged 'safe' for widescreen broadcast. Joss Whedon stipulates that he'd prefer the widescreen versions weren't out there at all, but in today's TV climate I'd say that's almost impossible. My understanding of the TNG situation at least is that they
could have struck off widescreen versions, but that they'd have had similar problems as the Buffy episodes do, in other words things like set dressing being visible when it shouldn't be and so forth.
So yeah. It's got more to do with the way the action is ''blocked'' than it does with anything more techical. The directors tended to block their scenes according to the 4:3 ratio, which meant that there's all sorts of off-screen badness that would become obvious if they ever struck off 16:9 prints. They'd have to digitally alter every frame to remove this stuff, which would be a whole headache of extra cost in itself...