CV for games 'no longer available on the Steam store'?

I think SG grabs the current Steam price, and if that's 0 / not existing, it needs to be updated manually, based on the latest price.

If I remember well, your best course of action is to open a ticket under "add game to list" (many lists exist, but it's a relatively well defined category still), link the sub of the game - or the steamcommunity page of it, if the store page is disabled - and a link to a price-tracking site's page to the game, like isthereanydeal.
Then just tell in the ticket that the game has been removed, [this] is the last known USD price, so its value should be manually changed to that score, then wait.
It may take time, but this raises the attention of mods, link the (likely accurate) pages, and they can do their job the fastest possible, if special rank is required for it.

Also created a test GA and upon reviewing it the usual line at the bottom with the expected Real CV was missing so I'm a bit confused here.

Could you elaborate more? I've never seen that line.

About your inquiry, I think SG will take the last know price in its database.
For example, this game https://store.steampowered.com/app/205950/
Last known price is 7,99 $ according to Steam DB. A giveaway will cost 8 P to enter and yield 7,99 CV.

I gave away this game and SGTools agrees with my stats (not exact match, since SGTools uses P for calculation, not $).
https://www.sgtools.info/sent/ngoclong19

If a paid game that later becomes free, a giveaway of it will cost 0 P and yield 0 CV.

The missing line at the bottom of the Review Giveaway page is something that's added by ESGST setting 3.40 (Real CV Calculator). ESGST grabs live prices from Steam, and obviously it can't do so for delisted games.

I don't think there's a problem with SG itself as far as calculating CV. From what I understand, SG caches the last recorded price, so when a game gets delisted, that price sticks forever unless the mods override it manually.
You can tell what the last recorded price was and hence how much CV you'll get from the points cost displayed on the Review Giveaway page.

So, I'm going to contradict Adam and say that you don't have to open a support ticket for this :)