While I'm not going to take mod action on this yet - which, come to think of it, broke the system for a week the last time I tried - I agree that this is a good idea.
"Interstellar planet" is, as far as I can tell, far less common than "rogue planet". I searched arXiv, ADS, and even Google Ngrams; all seemed to show "rogue planet" used far more often than "interstellar planet" - which matches with my limited experience, to be honest. You won't see either show up in many scientific papers, given that exoplanets as a whole have only been a major research area for the past 25 years, but there's certainly a divide.
I'm reluctant to call either one a technical term, and so it's harder to back up either position in the comments to this question, where lurch stated that "interstellar planet" encompasses both "rogue planets" and sub-brown dwarfs, a curious class of objects somewhere between massive gas giants and, of course, low-mass brown dwarfs. If we assume that "interstellar planet" isn't a technical term, then I don't think this argument holds much weight, especially as sub-brown dwarfs don't really form in the same way as traditional planets - even rogue ones. I also don't think we need to worry about them at all. How many questions on Worldbuilding have dealt with $\sim10M_J$ sub-brown dwarfs? Exactly.
Another argument that's important to consider for tag synonyms is how people will think of the tag. This was briefly touched on when I suggested messing with the tag for the Solar System, and DaaaahWhoosh wrote
I don't trust the average Worldbuilding user to know which tag to use if we make a second one.
Basically, people aren't going to look into the details and try to figure out the difference if we have two different tags for "rogue planets" and "interstellar planets", if there's any difference at all (which I don't think there is; I don't agree with lurch's definition). They'll think they mean the same thing. It therefore makes more sense to just synonymize the two.
I'd say not; as interstellar planets is generally a catch-all terminology for any planet not within our Star's sphere of influence; not just rogue planets; but also planets in other Star Systems influences. The two elements are related, sure, but I don't think this warrants a synonym between
But that's just my opinion
Edit in response to comments about exoplanets: If you're going just by the OEDs definition (Link Here); which is "Adj. Occurring or situated between stars."; then perhaps you have an argument to be made for using the synonym.
However, the related article agreed that the tag of
exoplanet wasn't needed
At first glance I'd say yes:
It doesn't matter whether it is the same or something very similar. People will never be able to distinguish between terms with only a slight difference in meaning. Also I find the correct usage of tags not always easy anyway, there is no need for such an additional difficulties.
At second glance I look the terms up in Wikipedia.
There they are used as synonyms. That means I actually was wrong about my definition of rogue planets (though not on my definition of interstellar planets), and I apologise for that. That means, as far as I can tell, they definitely should be considered as synonyms.