In case you haven't checked lately, they updated the language out at
https://members.marriott.com/benefits/
NOTE: Members that reach 750 nights and 10 years at Platinum by December 31, 2018 will be grandfathered into Lifetime Platinum Premier Elite status (notified January 2019). In addition, members that achieve Marriott Rewards Lifetime Platinum under the legacy requirements by year end will also be grandfathered into Lifetime Platinum Premier Elite status. And, don’t forget that we’ll combine Lifetime activity across both Rewards and SPG toward qualification when members combine accounts in August. Beginning in 2019, qualification into Lifetime Platinum Premier Elite status will not be available. See FAQs for details.
So if you want to get into LTPPE status, you have until 12/31 to do so. There will not be an opportunity to do so after (unless they further change the program). Also, the combined nights and years at platinum will be used. My ambassador suggested I verify with Marriott that they would count my 2 years of Platinum from several years ago in addition to the 9 I have with SPG. They told me yes. The other option was to race out and get 75 MR nights by year end to "double qualify" in both programs this calendar year which I didn't really want to have to do.