How do I know the info is valid for v18?

Mark,

  Good question.  Since I have to refer to many kb's myself as well as online documentation, I always make it a habit to check what product/version it is for first before I review it.  

Take this kb as an example, it will usually state how to do something for v17 and also v18 in it: 
https://support.sophos.com/support/s/article/KB-000038081?language=en_US

For documentation online like the one below, notice it has the version of SFOS it is referring to in the URL.  Here it is referring to 18.5 of SFOS.
https://docs.sophos.com/nsg/sophos-firewall/18.5/Help/en-us/webhelp/onlinehelp/nsg/sfos/tasks/t_VPNIPsecSiteToSiteTroubleShootCommonErrors.html

There are many resources out there, but hopefully you find this helpful.

Should I start a list of every page I find that doesn't state a version anywhere visible, including the address line? How about a list of every page that doesn't provide instructions that actually work on the version I am running? My head is starting to hurt from all the dead links in the v18 how to list.

Hello Mark,

Thank you for contacting the Sophos Community.

If you have found any dead links, please share them with me so I can ask the pertinent team to check.

Regards,

Hello Mark,

Thank you for contacting the Sophos Community.

If you have found any dead links, please share them with me so I can ask the pertinent team to check.

Regards,