On the remove reviews subject: the only thing which can be linked somehow to the versions of the extension is the date when the comment was made, so filtering the review based on this criteria is unfortunately not feasible. The suggestion is interesting, and if was workable, I would support it fully.
Generally there are multiple causes why the current reviews system should be reworked in a healthier basis. A simple light saber cut a'la Obi Wan Kenobi is not possible. Let me enumerate couple of real world situations to see why is not a simple thing - and why all attempts to make this better have failed up to date.
1. The first one - see above: the reviews have a single piece of info you can use to link to a version of the extension.... you need to dig a lot to find out if this was made even for the version running on the current Joomla core. And then we didn't touched the more important point - if the review has been made for a recent version of the extension.
2. There where periods of history of JED when the handling of the reports related to extensions was... well, at best chaotic. As a result, we have still really useless/daunting reviews, or some revenge-reviews still up there. Check this:
- a FREE extension has got back in 2015 this review: "Too expensive for bad feautures". You can judge the value of this review... Why is still there, you can ask? Simply because there is one JED rule, which is saying that a review should be reported within X days to be checked by JED team and to have a decision if should stay or should go. Well, in this case, for various reasons the owner failed to meet the deadline AND when the review was reported the JED team of that time failed to solve the case - so the review is still there. What is the problem, you can say? Well, you can see this concrete "review" only after you see the details of the extension - most potential users don't even get there, because they aren't seeing the 5 stars in any other views....
3. Fake reviews - the JED is FULL of fake positive and negative reviews. There where real "review wars" between extension developers competing for same niche. In some times the results are still there. Past couple of years we did a pretty good job in stopping these, but the "ruins"of older wars are still there.
4. Mergers/takeovers - there are lot of extensions developed by someone, and currently continued by someone else. And I am not talking about forked extensions - from the point of view of reviews that's a clear situation. But what is the value of a review praising the customer support done years ago for a given extension developer, when the extension has been taken over by someone else? (Especially IF you know, that this was a hostile takeover, we have couple of examples.
5. Developers retired/deceased/moved to another CMS, but still having their extensions downloadable, otherwise conforming to the JED rules - so can't be removed - at least not yet...
And I am stopping here.
We don't have - JED NEVER HAD - the capacity to do a proper check of ALL extensions, ALL reviews. We are relying on couple of automated tools - like regularly checking via a bot if the posted links are still live - and ON THE COMMUNITY, on the reports received from the people. We are checking EVERY REPORT and we are giving a resolution on each of these. Yes, I know, we have couple (not too many) of tricky cases where the resolution is taking more, than the one week limit we are proposed to ourselves.
But that is simply not enough to find and to solve all the potential issues.
Definitively we don't have the resources to check one-by-one if a review has been made for the recent version of the extension - and Joomla core - or not.
And the age of review anyway does not tell you the whole story. As the age of the extension.
Let me illustrate this with a very personal example. I have in JED a little module I wrote in a hurry many years ago to solve a simple problem, and decided to publish it on the JED too. The module wasn't updated at least for 6 years for now,but works perfectly, does the simple task for what was written, and got couple of reviews over years (<5 in total). Should the reviews got for it when was first published stay or should go? Or even better, should the extension be marked as abandoned because wasn't updated for years?
Well, we need to take such decisions here. Picking the best - automated - solution is tricky. And inevitably, if you do this based on some criteria verified by a computer there will be false positives, unjustified losses too. But this mess should be solved somehow and the entire reviews and ratings mechanism should provide better info for the people seeking for an extension.
One thing is clear - we need a consensus when these decisions are taken. And this is just one problem we are facing - almost ALL JED rules have similar implications. Changing them overnight, based on an one man decision or by a small group's will MIGHT be effective, but one thing is sure, won't be democratic.
Regulating something via a democratic process is a long and painful experience. And you risk to be labeled as non-democratic during the process

So, removing the old reviews CAN BE part of the solution - can sweep out lot of problems - but also can lead to unwanted side effects. But, generally speaking, good and popular extensions, the ones maintained regularly are continuously receiving new reviews, so those won't be hurt. And that's the goal here - to promote quality, even if just by these simple actions.