Im trying to work out what rules still apply to white scars. I have watched lots of videos about the different space marine factions and white scars Play style appealed to me. Since getting the support codex I can't find the rule that lets them charge after falling back or advertising this being the main point of the videos I have watched as there are not very old I didn’t think it would be a redundant rule. So if anyone could shine some light on this rule it would be a great help.
3Be aware that "not very old" may still be "too old" right now, as WH40K 9th Edition has released in the past 2 weeks.– Philip KendallAug 3, 2020 at 9:36
To the best of my knowledge (I'm quite new to the game), the current edition of WH40k (9th, ca. Aug 2020) is largely compatible with the previous edition (8th, ca. June 2017), but not any of the earlier editions. If the videos you were watching are older than 2017 then they are very likely to be out of date.– James OtterAug 4, 2020 at 4:46
That said, I believe the trait you're looking for is called "Lightning Assault". I couldn't tell you where to find it (I'm not a Space Marine player and haven't read their codices), but it looks like it applies to any battleforged army consisting of only "White Scars" units– James OtterAug 4, 2020 at 4:55
James Otter's comment is right. The rule you're looking for the the Lightning Assault is the White Scar's "Chapter Tactic" and allows a Charge after Falling back or advancing.
It's covered in detail her Goonhamemr Review
The Supplement itself was first published in August 2019 according to its Amazon listing. But the rules for the Chapter Traits appear in the main Space Marine Codex.
Note that the linked articles are from the 8th Edition period, and Space Marines as GW's poster children are the most subject to frequent change. I'm unaware if these rules have been modified by the recent 9th Edition release.