I know Adobe has essentially released CS2 for free, i.e., giving away the license keys. Is it legal, if I owned CS4 at one point (but lost it), to download and use it?
This was a misunderstanding (and it hasn't been corrected in the first google search result, which confuses people even more).
The links were only for those who had purchased CS2 and needed to re-install CS2 now that the registration servers are offline.
Adobe is terminating the activation servers for CS2 and that for existing licensed users of CS2 who need to reinstall their software, copies of CS2 that don't require activation but do require valid serial numbers are available. You are only legally entitled to download and install with that serial number if you have a valid license to the product.