Hi there,
If you look at the parent stylesheet it states this: Foundation 6.2.3…I have no idea what version 99…that maybe something that a previous developer did or added? There is no version 99 of WP-Forge. Maybe they changed the number to keep the notice of updates from popping up, this is possible. I have seen this trick used numerous times.
WP-Forge has followed the versioning of Foundation since 5.2.2.2. Like I said all I see in the parent stylesheet is: “Foundation 6.2.3” and the date of “2016” in the child style sheet. So I would take this as the files have not been updated since then.
As per your question of being able to apply the latest version of WP-Forge to the site yes you can BUT you have to keep in mind there were a lot of changes made to Foundation since 6.2.3 version, which was released in 6-25-2016. You may want to test the waters on a demo site and see what will happen if you apply the latest version. Honestly I think the last version used on that site is 6.2.3.
Sorry I can’t be anymore help. What I would do is copy the site to a demo server or local server and then add the actual 6.2.3 copy and see what happens. If everything seems fine then I’d go ahead and apply the latest version and make the necessary changes. Again, I would tread lightly because the changes between the two are very significant.
Honestly I would just leave it the way it is, but I would def check and see if the 6.2.3 version is the last version used.