Just keep in mind that in almost all use cases of OpenWrt, the device and its running firmware are exposed to the perils of the open internet and/ or the wireless range (which, in urban environments is easily in range of 20-60 households of strangers). Keeping your firmware updated is essential to keep your home network secure (to the extent possible), even if commercial vendors (and their unsuspecting users) don't do a good job about updates, there's little reason why OpenWrt should follow suit (keep in mind, OpenWrt users intentionally walked the extra mile to get it installed on their device in the first place).
While I realize that older, sub-spec, devices falling out of support is a problem for users, that shouldn't become an excuse to remain on an unsupported old version - security shouldn't be up for negotiations (beyond a couple of weeks/ months). Especially considering that most of the affected devices are about a decade old and came from the bottom of the range to begin with (802.11n, usually 2.4 GHz only, not keeping up with VDSL2 and up, etc.) - sure, they may still be good enough, but security is part of the tally.
With the wiki having a revision history for each page, removing information doesn't imply that it's totally gone - you can view the documentation from around the time when your firmware version was still golden). Yes, this is slightly more effort on the side of the user, to walk back memory lane for finding information about long-unsupported devices, but it keeps the contemporary lean and tidy, making it easier to understand without too many
if-then-else constructs based on version numbers. As mentioned before, there will always be some overlap by the nature of things, but I wouldn't declare it an art form (just drop the obsolete stuff to the revision history roundabout half-way through).