Back at the San Diego Comic-Con 2019, Marvel had clearly put out their full roster for MCU Phase 4, but they did tease some films that would be coming later in Phase 5. With all this speculation on when the Marvel Fox characters will join the MCU, Deadpool's Ryan Reynolds posts up a possible teaser for his involvement:
Of course, it's more likely that Reynolds is just celebrating the anniversary of when the test footage for the Deadpool film leaked. Though Deadpool is a box office giant now, Reynolds was said to have tried for years to get the movie done with Fox still feeling unsure. With the leaked footage and fan reaction though, Fox finally gave the film the greenlight, and the rest is history.
We still don't know who leaked the footage officially, but almost everyone is convinced that it was either Reynolds or director Tim Miller who did it. If anything, it was a huge gamble that paid off. Deadpool is now the highest R-rated movie of all time, and it's rumored that Marvel is currently prioritizing his entry into the MCU before the X-Men.
As for mentioning "Phase 5", I have a feeling that Reynolds knew that it would definitely spark discussion among fans. Is Deadpool coming in MCU Phase 5? Until we get an official announcement from Marvel, all we can really do is wait.
As of now, the last film of Phase 3, Spider-Man: Far from Home, is currently in theaters.