Is it OK to post links to such mods?
We have already agreed that we are not lawyers. We don't have the expertise to say what goes and what doesn't go against local regulations, game licenses and whatnot.
Did you read all of the EULA? Did you study contract law to know what can apply and what can't? Not your regulations, but the asker's state's or country's, obviously.
Mods are integral part of gaming and their discussion shouldn't be hindered by the general sadness of two screenfuls of comments on their "usefulness", as if any of us here had any objective data on it. If you have legal worries, do point them out, but they're just that - worries.
As Jeff said, we don't need to care about legality. The burden of checking legality falls on the individuals, if they need assistance with that they can ask a lawyer.
My opinion -- unless the site is clearly illegal and hosting "warez", it's OK to link to these sorts of grey area "abandonware" sites.
The legality of following the link and downloading abandonware is a decision for the person behind the mouse click, and the person running those sites, not us.
Do not take a share of this responsibility. It's not our place on the internet.
I also believe expert answers on this topic would be incredibly educational for the gaming population at general -- but that's just my opinion, not the direct consequence of something we do agree on -- we are not lawyers. Downvote the questions to hell if you want, ignore them, make your loathe heard, but they're still useful to somebody.
It really depends on what a "mod" is. For example, there are some games, often open source ones but also a lot of popular FPS games, where "mods" are very common and encouraged. Things like new maps, different game modes, etc., I don't think these are problematic to have on the site.
Also take into consideration Jeff's answer here. If the subject is actual illegal modification, or any sort of modification that is against the terms of service of a game, then I recommend not linking. But if it is just an add-on or patch that can be applied without any trouble, then I think it should be fine. I think these kinds of mods are less open to being in "gray areas" than other things.
That said, analyze these on a case-by-case basis because maybe they are pretty gray for some situations. If you as an answerer can't discern whether or not a mod is legal or not, I think it's better to keep your hands clean and avoid posting it. Modifications are a much different beast than piracy (pretty clear) and emulation (never been clear), not because they are legally gray but because there's a lot you can define as a "mod".
I think if the game is known to be open to mods (source games, for instance) then links are ok. If the game has a closed approach to mods and said mods must bypass security features (such as in Modern Warfare 2) then they require closer attention.
Generally speaking, I think that if the legality of such mods is in question, we can only name the mod without providing a link.